Method and apparatus for performing rewind structural verification of retimed circuits driven by a plurality of clocks

ABSTRACT

A method for designing a system on a target device includes performing register retiming on an original design for the system to generate a retimed design. The retimed design is verified to determine whether it is structurally correct by performing a plurality of iterations of register retiming on the retimed design, wherein each iteration accounts for the register retiming of registers in the system driven by a different clock.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/718,424 filed Sep. 27, 2017, entitled, “Method And Apparatus ForPerforming Rewind Structural Verification Of Retimed Circuits Driven ByA Plurality Of Clocks”, which is a continuation-in-part of and claimsthe benefit under Title 35 United States Code, Section 120 of U.S.patent application Ser. No. 15/079,390 filed on Mar. 24, 2016, nowissued U.S. Pat. No. 9,824,177 entitled “Method and Apparatus forVerifying Structural Correctness in Retimed Circuits”, both of which arehereby incorporated by reference.

FIELD

Embodiments of the present disclosure relate to tools for designingsystems on target devices. More specifically, embodiments of the presentdisclosure relate to a method and apparatus for verifying structuralcorrectness in retimed circuits.

BACKGROUND

Target devices such as field programmable gate arrays (FPGAs),structured application specific integrated circuits (ASICs), and ASICsare used to implement large systems that may include million of gatesand megabits of embedded memory. The complexity of a large system oftenrequires the use of electronic design automation (EDA) tools to createand optimize a design for the system onto physical target devices. Amongthe procedures performed by EDA tools in a computer aided design (CAD)compilation flow is hardware description language (HDL) compilation. HDLcompilation involves performing synthesis, placement, routing, andtiming analysis of the system on the target device.

Functional verification is a procedure that may also be performed duringHDL compilation by EDA tools. Functional verification is used to ensurethe functional correctness of implemented circuits. When used, more than70% of a design cycle may be spent performing functional verification.Techniques that may be used for functional verification includesimulation and formal verification. Simulation is typically used toverify the correctness of Register-Transfer-Level (RTL) circuitdescription against design intent. Constrained random simulation is atechnique that may be used to reduce simulation time and to increasefunctional coverage efficiency. Constrained random simulation has beenshown to be effective in identifying bugs early in a design cycle. OnceRTL is implemented using EDA tools, formal verification may be used toverify the correctness of a circuit against the RTL description. Formalverification can be a computationally difficult problem to solve as itseeks to mathematically prove that the two circuits being compared haveidentical functional behavior. To cope with this complexity, some formalverification techniques and tools in the industry are combinationalverification tools. Combinational verification tools use primary outputsand register boundaries as compare points for the two circuits beingcompared for equivalency.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of embodiments of the present disclosure areillustrated by way of example and are not intended to limit the scope ofthe embodiments of the present disclosure to the particular embodimentsshown.

FIG. 1 is a flow chart illustrating a method for designing a system on atarget device according to an exemplary embodiment of the presentdisclosure.

FIGS. 2A-2C illustrate an example of register retiming according to anexemplary embodiment of the present disclosure.

FIG. 3 illustrates a retiming graph according to an exemplary embodimentof the present disclosure.

FIG. 4 is a flow chart illustrating a method for performing verificationof a retimed circuit according to an exemplary embodiment of the presentdisclosure.

FIG. 5 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit according to an exemplary embodiment ofthe present disclosure.

FIGS. 6A-6B illustrate an example of a pipeline sequential circuit and aretimed pipeline sequential circuit.

FIG. 7 illustrates an example of changed and unchanged flip-flops afterregister retiming according to an exemplary embodiment of the presentdisclosure.

FIG. 8 is a flow chart illustrating a method for verifying initial stateequivalence of unchanged flip-flops in a retimed circuit according to anembodiment of the present disclosure.

FIG. 9 is a flow chart illustrating a method for determining a leftindex for an edge in an original circuit and a retimed circuit accordingto an embodiment of the present disclosure.

FIG. 10 is a flow chart illustrating a method for determining a rightindex for an edge in an original circuit and a retimed circuit accordingto an embodiment of the present disclosure.

FIG. 11 illustrates an example of changed flip-flops after registerretiming according to an exemplary embodiment of the present disclosure.

FIG. 12 is a flow chart illustrating a method for verifying initialstate equivalence of changed flip-flops in a retimed circuit accordingto an embodiment of the present disclosure.

FIG. 13 is a flow chart illustrating a method for identifying comparepoints and performing bounded sequential logic simulation according toan embodiment of the present disclosure.

FIG. 14 illustrates a block diagram of a computer system implementing asystem designer according to an exemplary embodiment of the presentdisclosure.

FIG. 15 is a block diagram of a system designer according to anexemplary embodiment of the present disclosure.

FIG. 16 illustrates an exemplary target device according to an exemplaryembodiment of the present disclosure.

FIG. 17 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit using equivalence classes according toan embodiment of the present disclosure.

FIG. 18 is a flow chart illustrating a method for identifying randomvariables for an equivalence class according to an exemplary embodimentof the present disclosure.

FIGS. 19A and 19B illustrate an example of an original design and aretimed design according to an exemplary embodiment of the presentdisclosure.

FIGS. 20A and 20B illustrate an example of retiming graphs of theoriginal design and the retimed design and an example of how randomvariables may be selected for an equivalence class according to anexemplary embodiment of the present disclosure.

FIG. 21 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit driven by a plurality of clocksaccording to an embodiment of the present disclosure.

FIG. 22 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit driven by a plurality of clocks asimpacted by a specific clock according to an embodiment of the presentdisclosure.

FIG. 23 illustrate an example of an original design driven by aplurality of clocks according to an exemplary embodiment of the presentdisclosure.

FIGS. 24A and 24B illustrate retiming graphs of the original design whenperforming rewind structural verification with respect to a first clockand a second clock.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, specificnomenclature is set forth to provide a thorough understanding ofembodiments of the present disclosure. It will be apparent to oneskilled in the art that specific details in the description may not berequired to practice the embodiments of the present disclosure. In otherinstances, well-known circuits, devices, procedures, and programs areshown in block diagram form to avoid obscuring embodiments of thepresent disclosure unnecessarily.

A method and apparatus for verifying structural correctness in retimedcircuits is disclosed. According to an embodiment of the presentdisclosure, a method in which an original circuit is transformed to aretimed circuit is reverse engineered. Using new constraints, aprocedure is performed to determine whether the retimed circuit may beretimed back to the original circuit by solving for retiming labels. Theretiming labels identify a number of flip-flops that are repositionedand a direction the flip-flops are repositioned relative to a node in acircuit. If the procedure is successful, it is concluded that theretimed circuit's structural netlist is structurally correct. Thisprocedure may be referred to as rewind verification or rewind structuralverification.

FIG. 1 is a flow chart illustrating a method for designing a system on atarget device according to an exemplary embodiment of the presentdisclosure. The target device may be a field programmable gate array(FPGA), application specific integrated circuit (ASIC), a structuredASIC, or other device. According to one embodiment, the procedureillustrated in FIG. 1 may be performed by a computer aided design(CAD)/electronic design automation (EDA) tool implemented on a computersystem.

At 101, a design for the system is synthesized. The specification forthe system may be provided though a design entry tool. The specificationmay describe components and interconnections in the system. According toan embodiment of the present disclosure, the design entered may be inregister transfer level (RTL) in a hardware description language (HDL).Synthesis includes generating a logic design of the system to beimplemented by the target device. According to an embodiment of thepresent disclosure, synthesis generates an optimized logicalrepresentation of the system from an HDL design definition. Theoptimized logical representation of the system may include arepresentation that has a minimized number of functional blocks such aslogic gates, logic elements, and registers required for the system.Synthesis also includes mapping the optimized logical representation.Mapping includes determining how to implement logic gates and logicelements in the optimized logic representation with the types orcategories of resources available on the target device. The resourcesavailable on the target device may be referred to as “cells” or“components” and may include logic-array blocks, registers, memories,digital signal processing blocks, input output elements, and othercomponents. According to an embodiment of the present disclosure, anetlist is generated from mapping. This netlist may be an optimizedtechnology-mapped netlist generated from the HDL.

At 102, the system is placed. According to an embodiment of the presentdisclosure, placement involves placing the technology-mapped logicalsystem design on the target device. Placement includes fitting thesystem on the target device by determining which specific resources onthe target device are to be assigned to and implemented by thetechnology-mapped netlist determined during synthesis. Placement mayinclude clustering which involves grouping logic elements together toform the logic clusters present on the target device.

At 103, the placed design is routed. During routing, routing resourceson the target device are allocated to provide interconnections betweenlogic gates, logic elements, and other components on the target device.Routability optimization may also be performed on the placed logicdesign. According to an embodiment of the present disclosure, the goalof routability optimization is to reduce the amount of wiring used toconnect components in the placed logic design. Routability optimizationmay include performing fanout splitting, logic duplication, logicalrewiring, or other procedures. It should be appreciated that one or moreof the procedures may be performed on the placed logic design.

At 104, register retiming and verification is performed on the system.According to an embodiment of the present disclosure, register retimingimproves the performance of sequential circuit by repositioningflip-flops (registers) without changing the combinational elementsbetween flip-flops and/or input outputs (IOs) that have the worst delay.Reducing the delay on combinational paths is the goal of registerretiming. After register retiming, verification is performed on theretimed design for the system to confirm that retimed design isequivalent to the original design. It should be appreciated thatregister retiming and verification 104 may be performed during and/orafter synthesis 101, placement 102, and/or routing 103.

At 105, timing analysis is performed on the retimed design of the systemgenerated. According to an embodiment of the present disclosure, thetiming analysis determines whether timing constraints of the system aresatisfied.

At 106, assembly is performed. The assembly procedure involves creatinga data file that includes information determined by the proceduresdescribed at 101-105. The data file may be a bit stream that may be usedto program a target device. By programming the target with the datafile, components on the target device are physically transformed toimplement the system.

Referring back to 104, it should be appreciated that various approachesto register retiming may be taken. Min-period retiming may be performedwhere flip-flops are repositioned in a circuit to achieve the best delayto minimize a clock period of the circuit. Min-period retiming does notimpose a restriction on a number of flip-flops in the circuit afterregister retiming. Min-area retiming may be performed where flip-flopsare repositioned in the circuit to minimize a number of flip-flops inthe circuit. Min-area retiming does not impose a restriction on a clockperiod of the circuit after register retiming. Constrained min-arearetiming may be performed where flip-flops are repositioned in thecircuit to minimize a number of flip-flops in the circuit subject to auser-specified clock period constraint. A practical variant ofconstrained min-area retiming is the approach of minimizing a number offlip-flops in a circuit while achieving a best clock period that isclosest to a user-specified clock period constraint. It should beappreciated that a combination of these approaches may be taken whenperforming register retiming at 104. FIGS. 2A-2C illustrate an exampleof register retiming according to an embodiment of the presentdisclosure.

FIG. 2A illustrates an exemplary sequential circuit 200 according to anembodiment of the present disclosure. This sequential circuit 200 hassix combinational gates, G1, G2, G3, G4, G5, and G6 with delays of 1, 1,1, 2, 2, 2 respectively, as shown. The sequential circuit 200 also hasfour flip-flops, F1, F2, F3, F4 that are all positive edge-triggeredflip-flops clocked by the same clock CLK. The sequential circuit 200 has3 primary inputs A, B, and CLK, one primary output, O, and fanoutsreconverging on gates G3 and G6. The maximum combinational delay throughthis circuit is 6. One such path is F1→G1→G3→G4→G6→F4. The clock periodfor this circuit is dictated by this longest path delay of 6.

FIG. 2B illustrates a retimed sequential circuit 200′. The retimedsequential circuit 200′ has flip-flops F1 and F2 forward retimed throughgates G1, G2, and G3. Retimed sequential circuit 200′ has only 3flip-flops and the maximum combinational delay is 4. This is the minimumnumber of flip-flops that is achievable for this circuit.

FIG. 2C illustrates a further retimed sequential circuit 200″. Thesequential circuit 200′ from FIG. 2B has its clock period reduced bybackward retiming flip-flop F4 across gate G6. This backward-retimedcircuit is shown in FIG. 2C. Sequential circuit 200″ has a maximumcombinational delay of 2 for all input-to-flip-flop,flip-flop-to-flip-flop, and flip-flop-to-output paths. Since the worstdelay of a single combinational cell in this circuit is 2, this is theminimum delay that can be achieved. Hence the sequential circuit 200″ inFIG. 2C represents the min-period retiming solution.

A synchronous sequential circuit, such as the circuit shown in FIGS.2A-C, may include a plurality of combinational logic gates andflip-flops. When performing register retiming on a synchronoussequential circuit, the following assumptions may be made. Allflip-flops in the circuit are clocked by the same clock source with thesame edge relationship. Clock skew to all the registers are zero. Delaysof all combinational gates are fixed and do not depend on actual loadingseen by the gates. There are no asynchronous loops. Complex registersincluding load, synchronous clear, and clock enable may be modeled withsimple D flip-flops and associated combinational logic. All flip-flopshave a known power-up state that is configurable to either 0 or 1. Alllogic gates in the circuit can produce a 0 and 1 for some inputcombination of values, and no logic gate is a constant function.

According to an embodiment of the present disclosure, when performingregister retiming on the synchronous sequential circuit, the circuit ismodeled as a retiming graph G(V, E), where the vertices represent thecombinational logic gates and the edges represent the connection toother combinational logic gates, inputs or outputs of the circuittraversing through one or more flip-flops. Each edge has a correspondingweight that represents the number of flip-flops on that edge connection.

FIG. 3 illustrates a retiming graph 300 according to an exemplaryembodiment of the present disclosure. Retiming graph 300 represents thesynchronous sequential circuit 200 shown in FIG. 2A. As shown, everyfanout edge is modeled explicitly in the graph. The weights next to eachedge in the graph represent the number of flip-flops in that connection.For example, there exist two flip-flops on the path from the output ofgate G6 to the input of gate G5. This is modeled as an edge from G6 toG5 with a weight of 2.

Register retiming attempts to label every vertex, i, in a retiming graphwith a label r_(i) that represents the number of flip-flops that moveacross vertex i. Label r_(i) is an integer and can be positive ornegative. A positive (negative) value of r_(i) indicates the number offlip-flops that moved backward (forward) across vertex i as part of theretiming solution. The labels of the primary input and primary outputnodes are fixed at 0. A retiming label of 0 implies there is no movementof flip-flops across that vertex.

The weight of an edge from vertex u to vertex v may be represented byw_(uv), and the weight of the same edge after retiming be represented bynw_(uv). The relationship between these terms may be illustrated below.nw _(uv) =r _(v) +w _(uv) −r _(u)  (1)

A path p exists from vertex a to vertex b if there is a sequence ofvertices and edges from vertex a to vertex b, such that each vertex onthe path has as input a directed edge from the previous vertex on thepath. It should be appreciated that the path may be sequential orcombinational, meaning that the number of flip-flops on all the edges ina path may be ≥0. The weight of the path, w_(p), is the sum of theweights of all edges on the path. A combinational path has w_(p)=0. Theclock period of the circuit is determined by the worst delay for allcombinational paths in the circuit.

The following matrix relationships further illustrate how registerretiming is performed.

$\begin{matrix}{{W\left( {u,v} \right)} = {\min\limits_{{p\text{:}u}->v}\left\{ w_{p} \right\}}} & (2) \\{{D\left( {u,v} \right)} = {\max\limits_{{{p\text{:}u}->{v\mspace{14mu}{and}\mspace{14mu} w_{p}}} = {W{({u,v})}}}\left\{ d_{p} \right\}}} & (3)\end{matrix}$

The W matrix in relationship (2) records an entry for every pair (u, v)of vertices that have a path between them. The entry that is recorded isthe number of flip-flops on a path from u→v that has the minimum numberof flip-flops. This path has the minimum latency from u→v. For everypair of vertices (u, v), the D matrix in relationship (3) stores themaximum delay of the path from u→v whose flip-flop count was stored inthe W matrix.

When taking the min-period retiming approach, the following constraintsneed to be satisfied. After retiming, all edge weights need to benon-negative (nw_(uv)≥0). This allows relationship (1) to be representedwith the following relationship.r _(v) −r _(u) ≥−w _(uv)  (4)In addition, for a clock period, c, each path from u>v that has D(u,v)>c requires at least one register on it. This constraint isillustrated with the following relationship.r _(v) −r _(u) ≥−W(u,v)+1 ∀u→v such that D(u,v)  (5)

When taking the constrained min-area retiming approach, embodiments ofthe present disclosure attempts to find a retiming solution thatsatisfies a user-specified clock period with the minimum number ofregisters. The constraints for the retiming solution to be valid are thesame as those found in relationships (4) and (5). The completeformulation for the constrained min-area retiming for a target clockperiod of c is shown as follows.min Σ_(ucv)(|FI(v)|−|FO(v)|)r _(v)r _(v) −r _(u) ≥−w _(uv) ∀e _(uv) ∈Er _(v) −r _(u) ≥−W(u,v)+1 ∀D(u,v)>c  (6)

The computation of the W and D matrices is central to most retimingalgorithms. These matrices are primarily used to solve the constrainedmin-area retiming problem which involves adding new edges to theretiming graph that represent timing constraints. In addition to theoriginal “circuit” edges, additional “period” edges corresponding to thetiming constraints in relationships (5) and (6) are added to the graph.These period edges from u→v have a weight of W(u, v)−1.

FIG. 4 is a flow chart illustrating a method for performing verificationof a retimed circuit according to an exemplary embodiment of the presentdisclosure. The procedures illustrated in FIG. 4 may be used toimplement procedure 104 (shown in FIG. 1) in part.

At 410, the structural correctness of a retimed circuit is verified. Thecircuit may be a design implemented on a target device. According to anembodiment of the present disclosure, the structural correctness of theretimed circuit is verified by reversing how an initial circuit isretimed using constrained random simulation. New constraints areformulated to retime the retimed circuit back to the initial circuit. Ifthe procedure is successful, then the retimed structural netlisttransforms are determined to be correct.

At 420, it is determined whether structural correctness has beenverified. If structural correctness has been verified, control proceedsto 430. If structural correctness has not been verified, controlproceeds to 470.

At 430, unchanged flip-flops are identified and the initial stateequivalence of unchanged flip-flops in the retimed circuit are verified.

At 440, it is determined whether initial state equivalence existsbetween the unchanged flip-flops in the retimed circuit and the initialcircuit. If initial state equivalence exists between the unchangedflip-flops, control proceeds to 450. If initial state equivalence doesnot exist between the unchanged flip-flops, control proceeds to 470.

At 450, changed flip-flops and sequential compare points are identified,and the initial state equivalence of changed flip-flops in the retimedcircuit is verified. According to an embodiment of the presentdisclosure, this verification is achieved using bounded sequential logicsimulation.

At 460, it is determined whether initial state equivalence existsbetween the changed flip-flops in the retimed circuit and the initialcircuit. If initial state equivalence exists between the changedflip-flops, control proceeds to 480. If initial state equivalence doesnot exist, between the changed flip-flops, control proceeds to 470.

At 470, a message is generated indicating that verification wasunsuccessful.

At 480, a message is generated indicating that verification wassuccessful.

An original or initial circuit before retiming may be referred as C_(o).A retimed circuit may be referred to as C_(r). Structural correctnessmay be verified if it can be shown that C_(r) represents a correctstructural retiming of C_(o). A key aspect of the present disclosure isbased on the reversibility property for retimed circuits and that acorrect retiming operation is reversible. Forward retiming across a gateincludes moving a flip-flop from all inputs of a gate to the output ofthe gate. The flip-flops on the inputs of the gate need to be compatiblefor this operation to be structurally legal. Similarly, backwardretiming involves moving a flip-flop from its output to all its inputs.Both these retiming operations are reversible. A forward (backward)retimed circuit can be reversed using backward (forward) retiming on thesame combinational element.

Forward retiming across a fanout involves moving a flip-flop from afanout stem to the outputs of the gates on the fanout branches.Similarly, backward retiming across a fanout point involves movingcompatible flip-flops from the output of the fanout gates to the fanoutstem. Similar to retiming across gates, both these retiming operationsare reversible. A forward (backward) retimed circuit can be reversedusing backward (forward) retiming on the same fanout gates.

According to an embodiment of the present disclosure, the reversibilityproperty and relationship (1) are utilized to verify structuralcorrectness of a retimed circuit. The weights on each of the edgesbefore and after retiming are known. Since it is assumed that retimingdoes not change the combinational elements of a circuit, every edge inthe retimed circuit graph has a corresponding edge in the originalcircuit graph. Given the reversible property of retimed circuits, aretimed circuit that has been retimed correctly structurally can beretimed to the original circuit. For an edge from vertex u to vertex v,the weight on this edge in the retimed circuit, w_(uv), and the weightof this edge in the original circuit, nw_(uv), is known. For structuralverification purposes, the retiming labels, r_(u) and r_(v), may becomputed to satisfy relationship (1). The retiming labels or (rvariables) for all edges in the graph are simultaneously computed tosatisfy relationship (1) for all edges on the retiming graph.

If the retiming labels can be computed to satisfy relationship (1), itcan be concluded that the circuit was correctly retimed structurally.The value of the retiming labels on each combinational node vertex ofthe retiming label indicates how the original circuit was transformed tothe retimed circuit. Therefore, verification has reversed engineeredexactly which and how many flip-flops structurally moved in the circuitduring the retiming operation. If the retiming labels cannot be computedto satisfy relationship (1), it can be concluded that the circuit wasnot correctly structurally retimed, and this results in a verificationfailure.

By attempting to retime the retimed circuit back to the originalcircuit, embodiments of the present disclosure are solving only aspecial case of a global retiming problem. The global retiming problemexplores all retiming solutions and yields a best solution forobjectives being optimized which may be, for example, delay and area. Bysolving only a special case, the technique of the present disclosurerequires less time and fewer resources than the technique used forregister retiming. Thus the worst-case computational complexity of thistechnique is no worse than that of the retime itself.

FIG. 5 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit according to an exemplary embodiment ofthe present disclosure. The method may be referred to as rewindverification or rewind structural verification. The proceduresillustrated in FIG. 5 may be used to implement procedure 410 (shown inFIG. 4).

At 501, a first retiming graph is generated from an HDL description ofan original circuit. According to an embodiment of the presentdisclosure, the retiming graph models combinational nodes as verticeswith weights on edges representing a number of flip-flops betweencorresponding combinational nodes represented by that edge.

At 502, a second retiming graph is generated from an HDL description ofa retimed circuit. According to an embodiment of the present disclosure,the second retiming graph models the retimed circuit in a similar mannerthat the first retiming graph models the original circuit. The first andsecond retiming graphs may be traversed, and constraints may begenerated and solved in the manner described as follows.

At 503, the first and second retiming graphs are traversed to generateconstraints. According to an embodiment of the present disclosure, theconstraints may be processed by a constraint solver.

At 504, a first set of state variables is defined. The first set ofstate variables models weights for edges in a retimed circuit. A weightfor an edge in the retimed circuit represents a number of flip-flops onthe edge. As described above, structural correctness of a retimedcircuit is verified by retiming a retimed circuit and determiningwhether the resulting circuit is the original circuit.

At 505, a second set of state variables is defined. The second set ofstate variables models weights for edges in an original circuit. Aweight for an edge in the original circuit represents a number offlip-flops on the edge. As described above, structural correctness of aretimed circuit is verified by retiming a retimed circuit anddetermining whether the resulting circuit is the original circuit.

At 506, a third set of state variables is defined. The third set ofstate variables models retiming labels for inputs and outputs of thecircuit. A retiming label identifies a number of flip-flops that moveacross its associated vertex. The state variables identified at 504-506have values that do not change.

At 507, random variables are defined. The random variables modelretiming labels for nodes other than the inputs and outputs of thecircuit. The random variables model retiming labels for allcombinational nodes.

At 508, retiming constraints are defined. According to an embodiment ofthe present disclosure, for each edge in the retiming graph of thecircuit, a retiming constraint is modeled from relationship (1). Thestate variables and random variables defined at 504-507 are used toformulate the retiming constraints.

At 509 bound constraints are defined. According to an embodiment of thepresent disclosure, bound constraints may be used to limit a range forthe random variables.

At 510, a solution for the random variables is sought. According to anembodiment of the present disclosure, values for the random variablesare solved for given the state variables and constraints defined.Solutions for the random variables may be computed using an equationsolving routine or program.

At 511, it is determined if a solution for the random variables isfound. If a solution for the random variables is found, control proceedsto 512. If a solution for the random variables is not found, controlproceeds to 513.

At 512, an indication is generated to indicate that structuralcorrectness verification is successful.

At 513, an indication is generated to indicate that structuralcorrectness verification was unsuccessful.

In addition to solving for the retiming labels defined, the methodillustrated in FIG. 5 may also identify a maximum absolute value amongall retiming labels.

The following example illustrates how the verification method describedwith reference to FIGS. 4 and 5 may be performed on the sequentialcircuit illustrated in FIG. 2A and the retiming graph illustrated inFIG. 3 according to an embodiment of the present disclosure.SystemVerilog is used as the programming language in this example. Itshould be appreciated, however, that other programming languages ortools may be used to implement the methodology described. With referenceto FIG. 5, procedures 501-503 may be performed using knownmethodologies. The example below begins at procedure 504.

At 504, a first set of state variables is declared that models weightsfor edges in the retimed circuit 200″ shown in FIG. 2C. The weights forthe edges in the retimed circuit 200″ represent a number of flip-flopson the edges. Since all gates in this circuit have two inputs, thenotation we use is that the first input pin is a, the second input pinis b, and the output is z. For example, arrays and variables with a1 intheir names are referring to the a input of gate G1. The primary outputnode O is modeled with array and variable names that contain out.

// FF counts in retimed circuit integer win1, win2,  wout; integer wa1,wb1; integer wa2, wb2; integer wa3, wb3; integer wa4, wb4; integer wa5,wb5; integer wa6, wb6;

According to an embodiment of the disclosure, defining the statevariables that model weights for the edges of the retimed circuit mayinclude initializing the state variables as shown below.

// Setup FF counts of retimed circuit win1 = 0; win2 = 0; wa1 = 0; wb1 =0; wa2 = 0; wb2 = 0; wa3 = 0; wb3 = 0; wa4 = 1; wb4 = 1; wa5 = 1; wb5 =1; wa6 = 1; wb6 = 1; wout = 0;

At 505, a second set of state variables are defined that models weightsfor edges in the original circuit 200 shown in FIG. 2A. The weights forthe edges in the original circuit 200 represent a number of flip-flopson the edges.

// FF counts in original circuit integer new_wa1, new_wb1; integernew_wa2, new_wb2; integer new_wa3, new_wb3; integer new_wa4, new_wb4;integer new_wa5, new_wb5; integer new_wa6, new_wb6; integer new_wout;

According to an embodiment of the disclosure, defining the statevariables that model weights for the edges of the original circuit mayinclude initializing the state variables as shown below.

// Setup FF counts of original circuit  new_wa1 = 1;  new_wb1 = 1; new_wa2 = 1;  new_wb2 = 1;  new_wa3 = 0;  new_wb3 = 0;  new_wa4 = 0; new_wb4 = 2;  new_wa5 = 0;  new_wb5 = 2;  new_wa6 = 0;  new_wb6 = 0; new_wout = 1;

At 506, a third set of state variables are defined that models retiminglabel variables for inputs and outputs of the circuit 200 shown in FIG.2A (circuit 200″ shown in FIG. 2C).

//  Retiming  labels  for  primary  inputs  and primary outputs integerrin1, rin2, rout;

According to an embodiment of the disclosure, defining the statevariables that model the retiming label variables for inputs and outputsmay include initializing the state variables as shown below.

// Setup r variables for inputs and outputs  rin1 = 0;  rin2 = 0;  rout= 0;

At 507, random variables are defined to models retiming labels for nodesother than the inputs and outputs of the circuit 200 (or circuit 200″,since they have the same combinational nodes).

// Random variables  rand integer r1, r2, r3, r4, r5, r6;

At 508, retiming constraints are defined for each edge in the retiminggraph of the circuit shown in FIG. 3.

// Retiming constraints  new_wa1 == (r1 + wa1 − rin1);  new_wb1 == (r1 +wb1 − rin2);  new_wa2 == (r2 + wa2 − rin1);  new_wb2 == (r2 + wb2 −rin2);  new_wa3 == (r3 + wa3 − r1);  new_wb3 == (r3 + wb3 − r2); new_wa4 == (r4 + wa4 − r3);  new_wb4 == (r4 + wb4 − r6);  new_wa5 ==(r5 + wa5 − r3);  new_wb5 == (r5 + wb5 − r6);  new_wa6 == (r6 + wa6 −r4);  new_wb6 == (r6 + wb6 − r5);  new_wout == (rout + wout − r6);

At 509, bound constraints are defined to limit a range for the randomvariables. It should be appreciated that this procedure is optional.According to an embodiment of the disclosure, if it can be assumed thatmovement of flip-flops will not be required beyond a certain numberduring register retiming, values for the variable r_(i) may beconstrained to allow for more efficient computation. The following boundconstraints may be defined.

r1 >= −max_ffs; // −(2{circumflex over ( )}29 − 1) r1 <= max_ffs; r2 >=−max_ffs; r2 <= max_ffs; r3 >= −max_ffs; r3 <= max_ffs; r4 >= −max_ffs;r4 <= max_ffs; r5 >= −max_ffs; r5 <= max_ffs; r6 >= −max_ffs; r6 <=max_ffs;

In this example, max_ffs may be set to a value that will prune thesearch space for the constraint solver. According to an embodiment ofthe disclosure max_ffs may be set to a total number of flip-flops in thecircuit.

At 510, solutions for the random variables are sought given the definedstate variables and constraints using an equation solver. In thisexample, the following solutions were found for the random variables.

r1 = 1 r2 = 1 r3 = 1 r4 = 0 r5 = 0 r6 = −1

At 511, since a solution for the random variables is found, controlproceeds to 512 and an indication is generated to indicate thatstructural correctness verification is successful. Although this exampleshows an example of a correct structural retiming, the constraintssolver would fail if the retimer had performed an incorrect structuralretiming. According to an embodiment of the present disclosure, theconstraints solver identifies the minimal set of constraints that causedthe failure, enabling a debug procedure.

Since constraints are generated for each edge in the retiming graph,designs having millions of 2-pin nets may require millions ofconstraints. However, values for random variables become equal when thevalue for a new weight for an edge between nodes u and v, nw_(uv), isequal to the value of an old weight or original weight for the edge,w_(uv). This result can be observed from relationship (1) where whennw_(uv)=w_(uv), r_(v)=r_(u).

The value of a new weight for an edge may be equal to the value of anold weight for an edge when retiming does not touch the edge, or whenequal number of registers have entered and exited the edge as a resultof register retiming. By recognizing instances when the values of randomvariables are equal, consolidation of the random variables may beperformed where one of the random variables may be substituted foranother. Furthermore, when this approach is performed recursively, acollection of random variables having equivalent values may beidentified and assigned to an equivalence class. A single randomvariable may be used to represent every element (random variable) of theequivalence class. Some of the constraints that include variables in theequivalence class may be identified as being redundant in view of thesubstitution of random variables. The redundant constraints may beremoved and the constraint solver may only need to solve for theremaining variables using the remaining constraints.

Embodiments of the present disclosure may provide potential advantagessuch as reducing the size of the problem presented to the constraintsolver both in terms of the size of the file that is read and the numberof constraints that is to be processed. As a result, the time requiredfor performing rewind structural verification is reduced.

FIG. 17 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit using equivalence classes according toan embodiment of the present disclosure. The method illustrated in FIG.17 may be used in place of the method illustrated in FIG. 5, and may beused to implement procedure 410 (shown in FIG. 4).

At 1701, a first retiming graph is generated from an HDL description ofan original circuit. According to an embodiment of the presentdisclosure, the retiming graph models combinational nodes as verticeswith weights on edges representing a number of flip-flops betweencorresponding combinational nodes represented by that edge.

At 1702, a second retiming graph is generated from an HDL description ofa retimed circuit. According to an embodiment of the present disclosure,the second retiming graph models the retimed circuit in a similar mannerthat the first retiming graph models the original circuit. The first andsecond retiming graphs may be traversed, and constraints may begenerated and solved in the manner described as follows.

At 1703, the first and second retiming graphs are traversed to generateconstraints. According to an embodiment of the present disclosure, theconstraints may be processed by a constraint solver.

At 1704, a first set of state variables is defined. The first set ofstate variables models weights for edges in a retimed circuit. A weightfor an edge in the retimed circuit represents a number of flip-flops onthe edge. As described above, structural correctness of a retimedcircuit is verified by retiming a retimed circuit and determiningwhether the resulting circuit is the original circuit.

At 1705, a second set of state variables is defined. The second set ofstate variables models weights for edges in an original circuit. Aweight for an edge in the original circuit represents a number offlip-flops on the edge. As described above, structural correctness of aretimed circuit is verified by retiming a retimed circuit anddetermining whether the resulting circuit is the original circuit.

At 1706, a third set of state variables is defined. The third set ofstate variables models retiming labels for inputs and outputs of thecircuit. A retiming label identifies a number of flip-flops that moveacross its associated vertex. The state variables identified at1704-1706 have values that do not change.

At 1707, random variables are defined. The random variables modelretiming labels for nodes other than the inputs and outputs of thecircuit. The random variables model retiming labels for allcombinational nodes.

At 1708, retiming constraints are defined. According to an embodiment ofthe present disclosure, for each edge in the retiming graph of thecircuit, a retiming constraint is modeled from relationship (1). Thestate variables and random variables defined at 1704-1707 are used toformulate the retiming constraints.

At 1709 bound constraints are defined. According to an embodiment of thepresent disclosure, bound constraints may be used to limit a range forthe random variables.

At 1710, random variables are identified for designation into anequivalence class. According to an embodiment of the present disclosure,random variables are designated for an equivalence class if theycorrespond to an edge between a source node and a sink node where anumber of registers on the edge is unchanged after retiming. Anadditional random variable may be added to that equivalence class if theadditional random variable shares the same value with other randomvariables in that equivalence class, and if the additional randomvariable's corresponding node is connected to a node with a randomvariable in that equivalence class. In other words, an additional randomvariable may be added to that equivalence class if the additional randomvariable corresponds to a node sharing an edge with a node having arandom variable in that equivalence class, and if the shared edge has anumber of registers that are unchanged after retiming. After all randomvariables in a design are identified for designation into equivalenceclasses, the number of random variables may be reduced by substitutingthe random variables in an equivalence class with a single unique randomvariable.

It should be appreciated that all nodes in an equivalence class need notbe directly connected. For example, given that node A is connected tonode C, and node B is also connected to node C, where node C has twoinput nodes A and B. In this example, rA=rC and rB=rC. The equivalenceclass has {rA, rB, rC}, where A is not directly connected to B.

It should further be appreciated that in some embodiments, one need notgenerate all the constraints first and then reduce them based onequivalence classes of the random variables. Instead, the equivalenceclasses can be determined at or after procedure 1703, when traversingthe original and retimed graphs, and generating the constraints onlywith the reduced set of random variables and edges.

At 1711, in response to the consolidation/substitution of randomvariables, redundant constraints are identified. According to anembodiment of the present disclosure, the consolidation/substitution ofrandom variables at 1710 may render one or more of the retimingconstraints defined at 1708 to be redundant. The redundant constraintsmay be removed, resulting in a reduction in the number of constraints.

At 1712, a solution for the remaining random variables is sought. Theremaining random variables include the random variables that have notbeen substituted and may include new random variables assigned to anequivalence class. According to an embodiment of the present disclosure,the number of remaining random variables is fewer than the number ofrandom variables prior to consolidation/substitution. According to anembodiment of the present disclosure, values for the remaining randomvariables are solved for given the state variables and constraintsdefined. Solutions for the remaining random variables may be computedusing an equation solving routine or program. It should be appreciatedthat if a new random variable is used to represent an equivalence class,it is declared as a random variable. Alternatively, all random variablesin equivalence class may be replaced by one variable from the same classthat is already declared as a random variable.

At 1713, it is determined if a solution for the random variables isfound. If a solution for the random variables is found, control proceedsto 1714. If a solution for the random variables is not found, controlproceeds to 1715.

At 1714, an indication is generated to indicate that structuralcorrectness verification is successful.

At 1715, solutions are determined for the random variables defined priorto the consolidation/substitution of random variables using all of thedefined constraints. The original random variables and attempted to besolved using the original retiming constraints. This procedure isperformed in order to accurately map edge names to the retimed design toidentify a source of the unsuccessful structural correctnessverification.

At 1716, an indication is generated to indicate that structuralcorrectness verification was unsuccessful.

In addition to solving for the retiming labels defined, the methodillustrated in FIG. 17 may also identify a maximum absolute value amongall retiming labels.

FIG. 18 is a flow chart illustrating a method for identifying randomvariables for designation into an equivalence class according to anexemplary embodiment of the present disclosure. The method illustratedin FIG. 18 may be used to implement procedure 1710 (shown in FIG. 17).At 1801, a new edge in a retiming graph of an original circuit andretimed circuit that has not been evaluated is identified.

At 1802, it is determined whether a number of registers on the edge isunchanged after retiming. If the number of registers on the edge isunchanged after retiming, control proceeds to 1803. If the number ofregisters on the edge is changed after retiming, control proceeds to1807.

At 1803, the random variables corresponding to nodes defining the edgeare added to an equivalence class.

At 1804, it is determined whether a next edge is connected to a node ofthe edge having a number of registers unchanged. If a next edge isconnected to a node of the edge having a number or registers unchanged,control proceeds to 1805. If a next edge is not connected to a node ofthe edge having a number of registers unchanged, control proceeds to1806.

At 1805, it is determined whether a number of registers on the next edgeis unchanged after retiming. If the number of registers on the next edgeis unchanged after retiming, control proceeds to 1803. If the number ofregisters on the edge is changed after retiming, control proceeds to1804. It should be appreciated that when procedure 1804 is performedafter performing procedure 1803 or 1805, the next edge may be an edgethat is connected to any node having a random variable in theequivalence class.

At 1806, a new, unique random variable is designated for the randomvariables in the equivalence class. It should be appreciated thatinstead of designating a new, unique random variable for the randomvariables in the equivalence class, one of the random variables in theequivalence class may also be used be designated for all of the otherrandom variables in the equivalence class.

At 1807, it is determined whether all edges have been evaluated. If itis determined that all edges have been evaluated, control proceeds to1808. If it is determined that not all edges have been evaluated,control returns to 1801. According to an embodiment of the presentdisclosure, whenever control returns to 1801, any random variable addedto the equivalence class at 1803 is added to a new equivalence class.

At 1808, control terminates the procedure.

FIGS. 19A and 19B illustrate an example of an original design and aretimed design according to an exemplary embodiment of the presentdisclosure. FIG. 19A illustrates an original design where component A isconnected to component B, component B is connected to component C, andcomponent C is connected to component D. Register R is connected to theoutput of component D.

FIG. 19B illustrates a retimed design where register R from the originaldesign is moved backward from the output of component D to the inputs ofcomponent D. As a result of the retiming, register R₁ is coupled to afirst input of component D, and register R₂ is coupled to second inputof component D.

FIGS. 20A and 20B illustrate an example of a retiming graph of theoriginal design and the retimed design shown in FIGS. 19A and 19B, andan example of how random variables may be selected for an equivalenceclass according to an exemplary embodiment of the present disclosure.Components A-D are shown as nodes. Each edge in retiming graph has aweight value which represents a number of registers on the edge. Asshown in FIG. 20A, the edge at the output of node D has a weight of 1.As shown in FIG. 20B, the edges at the input of node D have the weightof 1 to reflect the retiming of register R from FIGS. 19A and 19B.

According to an embodiment of the present disclosure, random variablesare designated for an equivalence class if they correspond to an edgebetween a source node and a sink node where a number of registers on theedge is unchanged after retiming. An additional random variable may beadded to that equivalence class if the additional random variable sharesthe same value with other random variables in that equivalence class,and if the additional random variable's corresponding node is connectedto a node with a random variable in that equivalence class. In otherwords, an additional random variable may be added to that equivalenceclass if the additional random variable corresponds to a node sharing anedge with a node having a random variable in that equivalence class, andif the shared edge has a number of registers that are unchanged afterretiming. Evaluating the weights on the edges of nodes A-D, it can bedetermined that the random variable for nodes A through C belong in thesame equivalence class. As such, r_(A)=r_(B)=r_(C).

It should be appreciated that the techniques described may also be usedto perform rewind structural verification on a design driven by multipleclocks. Initially, a retiming graph may be generated for an originaldesign and a retimed design without weights assigned to edges. For eachclock, c_(i), in the design, a retiming graph for the original designand a retiming graph for the retimed design is generated where a weighton each edge, e_(j), is set, where the weight represents a number ofregisters driven by the clock Additional constraints are generated tohandle registers driven by different clocks on each edge to preventregister retiming to move a register driven by clock c_(i) over aregister driven by a different clock. Rewind verification may thenproceed by utilizing the procedures for verifying structural correctnessas described earlier with reference to FIGS. 5 and 17.

FIG. 21 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit driven by a plurality of clocksaccording to an embodiment of the present disclosure. The methodillustrated in FIG. 21 may be used in place of the method illustrated inFIG. 5, and may be used to implement procedure 410 (shown in FIG. 4).

At 2101, a retiming graph is generated from an HDL description of anoriginal circuit. According to an embodiment of the present disclosure,the retiming graph models combinational nodes as vertices.

At 2102, a retiming graph is generated from an HDL description of aretimed circuit. According to an embodiment of the present disclosure,the second retiming graph models the retimed circuit in a similar mannerthat the first retiming graph models the original circuit. The first andsecond retiming graphs may be traversed, and constraints may begenerated and solved in the manner described as follows.

At 2103, it is determined whether an unanalyzed is present. If anunanalyzed clock is present, control proceeds to 2104. If an unanalyzedclock is not present, control proceeds to 2108.

At 2104, weights are defined for edges on the retiming graph for theoriginal circuit. According to an embodiment of the present disclosure,a weight for an edge represents a number of registers driven by theclock identified at 2103 on that edge.

At 2105, weights are defined for edges on the retiming graph for theretimed circuit. According to an embodiment of the present disclosure, aweight for an edge represents a number of registers driven by the clockidentified at 2103 on that edge.

At 2106, rewind verification is performed for the current clock.According to an embodiment of the present disclosure, this includesperforming structural verification with new multi-clock constraints forthe current clock, as well as verification of unchanged and changedinitial states.

At 2107, it is determined whether verification passed for the currentclock. If verification has passed for the current clock, control returnsto 2103. If verification has not passed for the current clock, controlproceeds to 2108.

At 2108, control terminates the procedure.

According to an embodiment of the present disclosure, instead ofperforming procedure 2106 to evaluate structural correctness withrespect to registers driven by all clocks identified, control mayterminate the procedure at 2108 in response to determining structuralincorrectness with respect to registers driven by any one clockidentified.

FIG. 22 is a flow chart illustrating a method for verifying structuralcorrectness in a retimed circuit driven by a plurality of clocks asimpacted by a specific clock according to an embodiment of the presentdisclosure. The method illustrated in FIG. 22 may be used to implementprocedure 2106 (shown in FIG. 2) which may include performing structuralverification as well as verification of changed and unchanged initialstates for a current clock.

At 2201, the retiming graphs are traversed to generate constraints.According to an embodiment of the present disclosure, the constraintsmay be processed by a constraint solver. It should be appreciated thatwhen traversing the retimed and original circuits to generate theconstraints, in some embodiments, constraint minimization may also beapplied by identifying equivalence classes of random variables asdescribed.

At 2202, a first set of state variables is defined. The first set ofstate variables models weights for edges in a retimed circuit. A weightfor an edge in the retimed circuit represents a number of flip-flops onthe edge. As described above, structural correctness of a retimedcircuit is verified by retiming a retimed circuit and determiningwhether the resulting circuit is the original circuit.

At 2203, a second set of state variables is defined. The second set ofstate variables models weights for edges in an original circuit. Aweight for an edge in the original circuit represents a number offlip-flops on the edge. As described above, structural correctness of aretimed circuit is verified by retiming a retimed circuit anddetermining whether the resulting circuit is the original circuit.

At 2204, a third set of state variables is defined. The third set ofstate variables models retiming labels for inputs and outputs of thecircuit. A retiming label identifies a number of flip-flops that moveacross its associated vertex. The state variables identified at2202-2204 have values that do not change.

At 2205, random variables are defined. The random variables modelretiming labels for nodes other than the inputs and outputs of thecircuit. The random variables model retiming labels for allcombinational nodes. It should be appreciated that these could be areduced set of random variables depending on the results of theminimization procedures described.

At 2206, retiming constraints are defined. According to an embodiment ofthe present disclosure, for each edge in the retiming graph of thecircuit, a retiming constraint is modeled from relationship (1). Thestate variables and random variables defined at 2202-2205 are used toformulate the retiming constraints.

At 2207 bound constraints are defined. According to an embodiment of thepresent disclosure, bound constraints may be used to limit a range forthe random variables.

At 2208 multi-clock constraints are defined. According to an embodimentof the present disclosure, multi-clock constraints are generated toaddress registers driven by different clocks on each edge and to preventregister retiming to move a register driven by a clock over a registerdriven by a different clock. According to an embodiment of the presentdisclosure one or more multi-clock constraints are generated for eachedge on the retiming graph for the design, wherein the multi-clockconstraint prevents a register driven by a first clock to be moved tothe other side (past) a register driven by a second clock.

At 2209, a solution for the random variables is sought. According to anembodiment of the present disclosure, values for the random variablesare solved for given the state variables and constraints defined.Solutions for the random variables may be computed using an equationsolving routine or program.

At 2210, it is determined if a solution for the random variables isfound. If a solution for the random variables is found, control proceedsto 2211. If a solution for the random variables is not found, controlproceeds to 2212.

At 2211, an indication is generated to indicate that structuralcorrectness verification is successful.

At 2212, an indication is generated to indicate that structuralcorrectness verification was unsuccessful.

According to an alternate embodiment of the present disclosure, at 2210,if structural verification is successful, verification of unchangedinitial states for the current clock may be performed. If verificationof unchanged initial states fails, control proceeds to 2212. Ifverification of unchanged initial states succeeds, verification ofchanged initial states may be performed. If verification of changedinitial states succeeds, control proceeds to 2211. Otherwise, controlproceeds to 2212.

In addition to solving for the retiming labels defined, the methodillustrated in FIG. 22 may also identify a maximum absolute value amongall retiming labels. It should be appreciated that the techniquesdescribed with reference to FIGS. 17, 21, and 22 may be implementedtogether. For example, when verifying structural correctness in aretimed circuit driven by a plurality of clocks, as described in FIGS.21 and 22, equivalence classes may be identified for random variablesand redundant constraints may be removed, as described in FIG. 17, tosimplify the verification procedure.

FIG. 23 illustrate an example of an original design driven by aplurality of clocks according to an exemplary embodiment of the presentdisclosure. As shown in FIG. 23, registers 2301-2302 driven by a firstclock, c1, is on an edge between nodes X and A. Registers 2303-2305driven by the first clock, c₁, and registers 2311-2312 driven by asecond clock, c₂, is on an edge between nodes A and B. Registers2306-2307 driven by the first clock, c₁, is on an edge between nodes Band Y.

FIG. 24A illustrates a retiming graph of the original design whenperforming rewind structural verification with respect to the firstclock c₁. As shown, there is a weight of 2 on the edge between nodes Xand A to represent registers 2301-2302. There is a weight of 3 on theedge between nodes A and B to represent registers 2303-2305. There is aweight of 2 on the edge between nodes B and Y to represent registers2306-2307.

When deriving multi-clock constraints, it should be recognized that whenperforming retiming, registers driven by the first clock c₁ should notbe moved over or past registers 2311 and 2312 driven by the second clockc₂. To prevent registers 2303-2304 from moving to the other side ofregister 2311 or 2312 during retiming, the constraint r_(B)≥−1 isgenerated. To prevent register 2305 from moving to the other side ofregister 2311 or 2312 during retiming, the constraint r_(A)≤2 isgenerated.

FIG. 24B illustrates a retiming graph of the original design whenperforming rewind structural verification with respect to the secondclock c₂. As shown, there is a weight of 0 on the edge between nodes Xand A to represent the absence of registers. There is a weight of 2 onthe edge between nodes A and B to represent registers 2311-2312. Thereis a weight of 0 on the edge between nodes B and Y to represent theabsence of registers.

When deriving multi-clock constraints, it should be recognized that whenperforming retiming, registers driven by the second clock c₂ should notbe moved over or past registers 2303-2304 or register 2305 driven by thefirst clock c₁. To prevent registers 2311-2312 from moving to the otherside of register 2303 or 2304, or to prevent any registers driven byclock c2 to retime into the edge from A to B from the X to A edge duringretiming, the constraint r_(A)=0 is generated. To prevent registers2311-2312 from moving to the other side of register 2305, or to preventany registers driven by clock c2 to retime into the edge from A to Bfrom the B to Y edge during retiming, the constraint r_(B)=0 isgenerated.

Referring back to 430 in FIG. 4, after structural correctness has beenverified, initial state equivalence of unchanged flip-flops may also beverified. Retimed circuits do not necessarily demonstrate the samesequential behavior as its original or initial circuit for all possibleinitial state conditions. The following example illustrates thisphenomenon.

FIG. 6A illustrates an example of a pipeline sequential circuit 600. Thepipeline sequential circuit 600 is a 2-stage pipeline circuit that maybe initialized with a single vector a=0, b=1. For all possible initialstates of flip-flops F1 and F2, this vector with a single clock cycleproduces and output of h=0.

FIG. 6B illustrates the pipeline sequential circuit 600 retimed as 600′.As shown, flip-flop F1 is repositioned forward onto its fanout branchesand is illustrated as F1 a and F1 b in FIG. 6B. For an initial state F1a=0, F1 b=1, and F2=1, and an initial vector a=0, b=1, the output h=1results. This initial state behavior cannot be observed in the originalcircuit 600 illustrated in FIG. 6A. As such, by definition of strictsequential equivalence, the pipeline sequential circuit 600 and retimedpipeline sequential circuit 600′ are not identical. To overcome thisissue, we assume that all flip-flops in the target device have adeterministic power-up initial state, that is programmable to 0 or 1. Aretimer for such a device architecture will then determine new initialpower-up states for the retimed flip-flops, based on the initialpower-up state of the corresponding flip-flops in the original circuitand the logic functionality of the combinational logic across which theflip-flops got retimed.

According to an embodiment of the present disclosure, results generatedfrom verifying structural correctness of a retimed circuit may be usedto determine whether initial state computations for flip-flops areperformed correctly. Flip-flops may be categorized as unchanged orchanged. Unchanged flip-flops are flip-flops that do not move duringretiming, whereas changed flip-flops are flip-flops that move duringretiming.

FIG. 7 illustrates an example of changed and unchanged flip-flops afterregister retiming according to an exemplary embodiment of the presentdisclosure. During retiming, flip-flops may move across combinationalelements A and B. In this example, flip-flops 701-702 move from aninitial position from the right of element A to a new position to theleft of element A or vice versa. Similarly, flip-flops 711-712 move froman initial position from the left of element B to a new position to theright of element B, or vice versa. Flip-flop 721 did not move duringretiming. Flip-flops 701-702 and 711-712 are considered changedflip-flops. Flip-flop 721 is an unchanged flip-flop.

As described with reference to verifying structural correctness, anoriginal or initial circuit before retiming may be referred as C_(o).Embodiments of the present disclosure verify that unchanged flip-flopsin a retimed circuit C_(r) have the same initial power-up state valuesas original circuit C_(o). Results from the structural verificationprocedure described are used to identify the unchanged flip-flops and toperform the verification of equivalence of initial power-up state valuesof unchanged flip-flops in the retimed circuit. Specifically, thecomputed retiming labels in conjunction with old and new weights of eachedge in the retiming graph are utilized.

It should be appreciated that the initial state values for flip-flops oneach edge in an original and retimed circuit may be represented asseparate arrays of values. For each such array on an edge from source uto destination v, using retiming labels r_(u) and r_(v), respectively,and original and retimed weights o_(w) and r_(w), a left index l_(i) andright index l_(r) are computed. If l_(i) is less than or equal to l_(r),it is concluded that the values in the array between l_(i) and l_(r)indices represent unchanged flip-flops. The values of the correspondingunchanged flip-flops in the array representing the retimed circuit andthe array representing the original circuit may be compared to ensurethat they are identical. If the values are not identical, a verificationfailure occurs and an error message is generated identifying theconnection where the mismatch resides.

FIG. 8 is a flow chart illustrating a method for verifying initial stateequivalence of unchanged flip-flops in a retimed circuit according to anembodiment of the present disclosure. The procedures illustrated in FIG.8 may be used to implement procedure 430 (shown in FIG. 4). It should beappreciated that prior to performing the procedures in FIG. 8, a firstretiming graph is generated from an HDL description of an originalcircuit and a second flow chart is generated from an HDL description ofa retimed circuit. According to an embodiment of the present disclosure,the retiming graphs model combinational nodes as vertices with weightson edges representing a number of flip-flops between correspondingcombinational nodes represented by that edge. The first and secondretiming graphs may be traversed, and constraints may be generated andsolved in the manner described as follows.

At 801, variables that model initial states of registers in an initialcircuit and a retimed circuit are declared. According to an embodimentof the present disclosure, the variables may be represented as staticarrays for each edge in the original and retimed circuits. An initialstate of a register is the state of the register at power-up. The valuesfor the initial states or registers for the original circuit and theretimed circuit are known after register retiming.

At 802, variables that model weights in the original circuit and theretimed circuit are declared. According to an embodiment of the presentdisclosure, a weight for an edge in a circuit represents a number offlip-flops on the edge. The values for the weights for the originalcircuit and the retimed circuit are known after register retiming.

At 803, variables that model retiming labels are declared. According toan embodiment of the present disclosure, a retiming label identifies anumber of flip-flops that move across its associated vertex. The valuesfor the retiming labels are known after verification of structuralcorrectness. As such, they represent state variables for the purposes ofverifying the initial state equivalence of unchanged flip-flops.

At 804, variables that model indices that identify unchanged registersare declared. According to an embodiment of the present disclosure, foreach array of initial state variables declared at 801, two indices aredeclared a left index and a right index. For any edge, the two indicesare identified such that a range of values between the two indicesrepresent unchanged flip-flops, if any unchanged flip-flops exist on theedge.

At 805, the values for the indices of each edge are determined.According to an embodiment of the present disclosure, values for theindices are computed for edges that have a non-zero number of flip-flopsin the original and retimed circuits (a non-zero value for the weightson its edge). FIGS. 9 and 10 illustrate exemplary methods fordetermining the values of indices on an edge.

At 806, it is determined whether all edges have been analyzed. If notall edges have been analyzed, control proceeds to 807 to analyze a nextedge which has yet to be analyzed. If all edges have been analyzed,control proceeds to 812.

At 807, it is determined whether an unchanged flip-flop resides on anext edge. According to an embodiment of the present disclosure, anunchanged flip-flow is determined to reside on an edge if the followingconditions are both true. First, the weights on the edge in the originalcircuit and the retimed circuit are both non-zero. This would mean thatat least one flip-flop exists on the edge of the original and retimedcircuits. Second, the left index l_(i) is less than or equal to theright index r_(i) in both the original and retimed circuit.

At 808, it is determined whether an unchanged flip-flop resides on theedge. If it is determined that an unchanged flip-flop does not reside onthe edge, control returns to 806. If it is determined that an unchangedflip-flop resides on the edge, control proceeds to 809.

At 809, it is determined whether the initial states of the unchangedflip-flop(s) identified are equivalent. According to an embodiment ofthe present disclosure, the left and right indices for the edgesdetermined at 805 identify the initial states of a range of one or moreflip-flops that are unchanged in the original and retimed circuits. Thevalues of the initial states as defined at 801, may be compared todetermine whether the initial states of unchanged flip-flops areequivalent.

At 810, if the initial states of the unchanged flip-flop(s) identifiedare equivalent, control returns to 806. If the initial states of anunchanged flip-flop(s) identified are not equivalent, control proceedsto 811.

At 811, an indication is generated to indicate that verification ofinitial state equivalence of unchanged flip-flop(s) is unsuccessful. Itshould be appreciated that a message may also be generated to indicatethe exact edge where the mismatch occurred.

At 812, an indication is generated to indicate that verification ofequivalent initial states of changed flip-flop(s) is successful.

According to an embodiment of the present disclosure, after adetermination that the initial states of the unchanged flip-flop(s)identified are not equivalent, control may return to 806 and continue toanalyze all edges in the circuits. After the analysis is complete, anindication may be generated to indicate that verification of equivalentinitial states of unchanged flip-flop(s) is unsuccessful. The identityof the edge(s) with unchanged flip-flop(s) with incorrect initial statesand/or the identity of the flip-flop(s) with incorrect initial statesmay also be provided.

FIG. 9 is a flow chart illustrating a method for determining a leftindex for an edge in an original circuit and a retimed circuit accordingto an embodiment of the present disclosure. The method illustrated inFIG. 9 may be used to implement procedure 805 (shown in FIG. 8) in part.It should be appreciated that the method illustrated in FIG. 9 may beapplied to determine a left index for each edge in an original circuitand a retimed circuit, where the edge has a source u and a destinationv.

At 901, it is determined whether a value of a retiming label for asource of the edge is less than zero. If the value of the retiming labelfor the source of the edge is less than zero, control proceeds to 902.If value of the retiming label for the source of the edge is not lessthan zero, control proceeds to 904.

At 902, the left index for the edge in the original circuit is zero.

At 903, the left index for the edge in the retimed circuit is a negativevalue of the retiming label for the source of the edge.

At 904, it is determined whether the value of the retiming label for thesource of the edge is greater than zero. If the value of the retiminglabel for the source of the edge is greater than zero, control proceedsto 905. If the value of the retiming label for the source of the edge isnot greater than zero, then the value of the retiming label for thesource of the edge must be equal to zero, and control proceeds to 907.

At 905, the left index for the edge in the original circuit is the valueof the retiming label for the source of the edge.

At 906, the left index for the edge in the retimed circuit is zero.

At 907, the left index for the edge in the original circuit is zero

At 908, the left index for the edge in the retimed circuit is zero.

FIG. 10 is a flow chart illustrating a method for determining a rightindex for an edge in an original circuit and a retimed circuit accordingto an embodiment of the present disclosure. The method illustrated inFIG. 10 may be used to implement procedure 805 (shown in FIG. 8) inpart. It should be appreciated that the method illustrated in FIG. 10may be applied to determine a right index for each edge in an original(or initial) circuit and a retimed circuit, where the edge has a sourceu and a destination v.

At 1001, it is determined whether a value of a retiming label for adestination of the edge is greater than zero. If the value of theretiming label for the destination of the edge is greater than zero,control proceeds to 1002. If value of the retiming label for the sourceof the edge is not less than zero, control proceeds to 1004.

At 1002, the right index for the edge in an original circuit is a valueof the weight for the edge in the original circuit minus one.

At 1003, the right index for the edge in a retimed circuit is a value ofthe weight for the edge in the retimed circuit minus one minus a valuefor the retiming label for the destination of the edge.

At 1004, it is determined whether the value of the retiming label forthe destination of the edge is less than zero. If the value of theretiming label for the destination of the edge is less than zero,control proceeds to 1005. If the value of the retiming label for thedestination of the edge is not less than zero, then the value of theretiming label for the destination of the edge must be equal to zero,and control proceeds to 1007.

At 1005, the right index for the edge in an original circuit, is thevalue of the weight for the edge in the original circuit minus one plusthe value of the retiming label for the destination of the edge.

At 1006, the right index for the edge in the retimed circuit is thevalue of the weight for the edge in the retimed circuit minus one.

At 1007, the right index for the edge in an original circuit is thevalue of the weight for the edge in the original circuit minus one.

At 1008, the right index for the edge in the retimed circuit is a valueof the weight for the edge in the retimed circuit minus one.

The following example illustrates how the verification method describedwith reference to FIGS. 4, and 8-10 may be performed on the sequentialcircuit illustrated in FIG. 2A and the retiming graph illustrated inFIG. 3 according to an embodiment of the present disclosure.SystemVerilog is used as the programming language in this example. Itshould be appreciated, however, that other programming languages ortools may be used to implement the methodology described.

With reference to FIG. 2A, references by arrays and variables to aletter and number refer to an input from the referenced letter at gate(component) with the referenced number. For example, arrays andvariables with a1 in their names refer to the a input of gate G1. Theprimary output node O is modeled with array and variable names thatinclude out.

With reference to FIG. 8, at 801, variables that model initial states ofregisters in an original circuit and a retimed circuit are declared.According to an embodiment of the present disclosure, the variables maybe represented as static arrays for each edge in the original andretimed circuits. Referring to the sequential circuit 200 in FIG. 2A andits corresponding retiming graph 300 in FIG. 3, the size of the arraysfor each edge in the retiming graph 300 is set as the weight of thatedge in the graph. If the weight is 0, a dummy array of size 1 may becreated. For example, for an edge to a1, the following array may bedeclared.

bit o_ff_a1[1]; //Edge to a1 in original circuit has 1 flip-flop

Similarly, for an edge to b4, the following array may be declared.

bit o_ff_b4[2]; //Edge to b4 in original circuit has 2 flip-flops

An array identifying the initial states of registers for all edges inthe original sequential circuit 200 in FIG. 2A, and the retimedsequential circuit 200″ in FIG. 2C may be declared as follows. We usethe “o_” prefix to refer to variables in the original circuit and the“r_” prefix for variables in the retimed circuit.

bit o_ff_a1[1]; bit o_ff_b1[1]; bit o_ff_a2[1]; bit o_ff_b2[1]; bito_ff_a3[1]; bit o_ff_b3[1]; bit o_ff_a4[1]; bit o_ff_b4[2]; bito_ff_a5[1]; bit o_ff_b5[2]; bit o_ff_a6[1]; bit o_ff_b6[1]; bito_ff_out[1]; bit r_ff_a1[1]; bit r_ff_b1[1]; bit r_ff_a2[1]; bitr_ff_b2[1]; bit r_ff_a3[1]; bit r_ff_b3[1]; bit r_ff_a4[1]; bitr_ff_b4[1]; bit r_ff_a5[1]; bit r_ff_b5[1]; bit r_ff_a6[1]; bitr_ff_b6[1]; bit r_ff_out[1];

According to an embodiment of the disclosure, defining variables thatmodel initial states of registers in an original circuit and a retimedcircuit may include initializing the variables as shown below.

task set_initial_values( );   // Initial power-up   o_ff_a1[0] = 1;  o_ff_b1[0] = 1;   o_ff_a2[0] = 1;   o_ff_b2[0] = 1;   o_ff_b4[0] = 0;  o_ff_b4[1] = 0;   o_ff_b5[0] = 0;   o_ff_b5[1] = 0;   o_ff_out[0] = 0;  r_ff_a4[0] = 1;   r_ff_b4[0] = 0;   r_ff_a5[0] = 1;   r_ff_b5[0] = 0;  r_ff_a6[0] = 1;   r_ff_b6[0] = 0;

At 802, variables that model weights in the initial circuit and theretimed circuit are declared. Variables representing the weights for alledges in the original sequential circuit 200 and the retimed sequentialcircuit 200″ may be declared as follows.

 integer o_wa1, o_wb1, o_wa2, o_wb2, o_wa3, o_wb3, o_wa4, o_wb4, o_wa5,o_wb5, o_wa6, o_wb6, o_wout;  integer r_wa1, r_wb1, r_wa2, r_wb2, r_wa3,r_wb3, r_wa4, r_wb4, r_wa5, r_wb5, r_wa6, r_wb6, r_wout;

According to an embodiment of the disclosure, defining variables thatmodel weights in an original circuit and a retimed circuit may includeinitializing the variables as shown below.

 task set_weights(rewind_structural_constraints bar);  //New weights represents the weights in the original circuit   o_wa1 =bar.new_wa1;   o_wb1 = bar.new_wb1;   o_wa2 = bar.new_wa2;   o_wb2 =bar.new_wb2;   o_wa3 = bar.new_wa3;   o_wb3 = bar.new_wb3;   o_wa4 =bar.new_wa4;   o_wb4 = bar.new_wb4;   o_wa5 = bar.new_wa5;   o_wb5 =bar.new_wb5;   o_wa6 = bar.new_wa6;   o_wb6 = bar.new_wb6;   o_wout =bar.new_wout;   //Current weights represents the weights in the retimedcircuit   r_wa1 = bar.wa1;   r_wb1 = bar.wb1;   r_wa2 = bar.wa2;   r_wb2= bar.wb2;   r_wa3 = bar.wa3;   r_wb3 = bar.wb3;   r_wa4 = bar.wa4;  r_wb4 = bar.wb4;   r_wa5 = bar.wa5;   r_wb5 = bar.wb5;   r_wa6 =bar.wa6;   r_wb6 = bar.wb6;   r_wout = bar.wout;  endtask

At 803, variables that model retiming labels are declared. The valuesfor the retiming labels are known after verification of structuralcorrectness and may be declared as follows.

-   -   integer r1, r2, r3, r4, r5, r6, rin1, rin2, rout;

The retiming labels computed from structural verification may be negatedto derive true retiming labels to transform the original circuit to aretimed circuit. According to an embodiment of the disclosure, definingvariables that model retiming labels may include initializing thevariables as shown below.

task set_rs(foo bar);   // Negate the values of r, to get the   // realvalue that the retimer implemented   r1 = −bar.r1;   r2 = −bar.r2;   r3= −bar.r3;   r4 = −bar.r4;   r5 = −bar.r5;   r6 = −bar.r6;   rin1 =bar.rin1;   rin2 = bar.rin2;   rout = bar.rout;  endtask

At 804, variables that model indices that identify unchanged registersare declared. Variables representing the indices for all edges in theoriginal sequential circuit 200 and the retimed sequential circuit 200″may be declared as follows.

   integer o_a1_li, o_a1_ri, o_b1_li, o_b1_ri, o_a2_li, o_a2_ri,o_b2_li, o_b2_ri, o_a3_li, o_a3_ri, o_b3_li, o_b3_ri, o_a4_li, o_a4_ri,o_b4_li, o_b4_ri, o_a5_li, o_a5_ri, o_b5_li, o_b5_ri, o_a6_li, o_a6_ri,o_b6_li, o_b6_ri, o_out_li, o_out_ri;   integer r_a1_li, r_a1_ri,r_b1_li, r_b1_ri, r_a2_li, r_a2_ri, r_b2_li, r_b2_ri, r_a3_li, r_a3_ri,r_b3_li, r_b3_ri, r_a4_li, r_a4_ri, r_b4_li, r_b4_ri, r_a5_li, r_a5_ri,r_b5_li, r_b5_ri, r_a6_li, r_a6_ri, r_b6_li, r_b6_ri, r_out_li,r_out_ri;

According to an embodiment of the disclosure, defining variables thatmodel indices in an initial circuit and a retimed circuit may includeinitializing the variables as shown below. For example, every left indexmay initially be set to −1 and every right index may initially be set to−2. The initialization of indices ensures that the right index is lessthan the left index on an edge in the event that no flip-flops reside onthe edge.

At 805, the values for the indices of each edge are determined.According to an embodiment of the present disclosure, in order todetermine the indices for the edge corresponding to the a input intogate G3, the following operations may be performed.

if (o_wa3 > 0 && r_wa3 > 0) begin   if (r1 > 0) begin    o_a3_li = r1   r_a3_li = 0;   end   else if (r1 < 0) begin    o_a3_li = 0;   r_a3_li = −r1;   end   else begin    o_a3_li = 0;    r_a3_li = 0;  end   if (r3 > 0) begin    ⊙_a3_ri = o_wa3 − 1;    r_a3_ri = r_wa3 − 1− r3;   end   else if (r3 < 0) begin    o_a3_ri = o_wa3 − 1 + r3;   r_a3_ri = r_wa3 − 1;   end   else begin    o_a3_ri = o_wa3 − 1;   r_a3_ri = r_wa3 − 1;   end end

It should be appreciated that the operations similar to thoseillustrated above may be used to determine the indices of other edges inthe original sequential circuit 200 and the retimed sequential circuit200″.

At 806, it is determined whether all edges have been analyzed. If alledges have not been analyzed, control proceeds to 807 to analyze a nextedge which has yet to be analyzed. If all edges have been analyzed,control proceeds to 812.

At 807, it is determined whether an unchanged flip-flop resides on anext edge. According to an embodiment of the present disclosure, anunchanged flip-flow is determined to reside on an edge if the followingconditions are both true. First, the weights on the edge in the originalcircuit and the retimed circuit are both non-zero. This would mean thatat least one flip-flop exists on the edge of the original and retimedcircuits. Second, the left index l_(i) is less than or equal to theright index r_(i) in both the original and retimed circuit.

At 808, it is determined whether an unchanged flip-flop resides on theedge. If it is determined that an unchanged flip-flop does not reside onthe edge, control returns to 806. If it is determined that an unchangedflip-flop resides on the edge, control proceeds to 809.

At 809, it is determined whether the initial states of the unchangedflip-flop(s) identified are equivalent. According to an embodiment ofthe present disclosure, the left and right indices for the edgesdetermined at 804 identify the initial states of a range of one or moreflip-flops that are unchanged in the original and retimed circuits. Thevalues of the initial states as defined at 801, may be compared todetermine whether the initial states of unchanged flip-flops areequivalent. This may be achieved by iterating through all the elementsbetween the left index and the right index (inclusively) of the initialstate values array for the edge in the original circuit, and comparingthe corresponding initial state value in the initial state values arrayfor the edge in the retimed circuit. An array lookup procedure may beutilized using the indices in the arrays of initial state values in theoriginal and retimed circuits

At 810, if the initial states of the unchanged flip-flop(s) identifiedare equivalent, control returns to 806. If the initial states of anunchanged flip-flop(s) identified are not equivalent, control proceedsto 811.

At 811, an indication is generated to indicate that verification ofinitial state equivalence of unchanged flip-flop(s) is unsuccessful. Amessage is also generated with the exact edge where the mismatchoccurred.

At 812, an indication is generated to indicate that verification ofequivalent initial states of changed flip-flop(s) is successful.According to an embodiment of the present disclosure, the followingexemplary function may be used to indicate verification of equivalentinitial states of changed flip-flop(s). The exemplary function appliesfor edge a3 from G1 to G3.

 if (o_wa3 > 0 && r_wa3 > 0 && o_a3_li <= o_a3_ri && r_a3_li <= r_a3_ri)begin    count = 0;    for (i = o_a3_li; i <= o_a3_ri; i++, count++)begin     if (o_ff_a3[i] != r_ff_a3[r_a3_li+count]) begin     $write(“Verification Error: Unchanged initial states for connectiona3 do not match”);      return 0;     end    end  end

It should be appreciated that the array iteration illustrated above maybe performed for all edges in the circuit. The function illustratedabove ensures that there a non-zero number of flip-flops are on the edgein the original and retimed circuits, and that the left index is lessthan the right index in the original and retimed circuits. The functioniterates the original circuit array between these indices and comparesthe corresponding values in the retimed array of values.

According to an embodiment of the present disclosure, results generatedfrom verifying structural correctness of a retimed circuit may also beused to determine whether initial state computations for changedflip-flops are performed correctly. The retiming labels identified fromperforming structural verification may be used to determine whichsignals in the original and retimed circuits correspond to each otherand appropriate compare points may be created to sequentially verifythese signals. Bounded sequential logic simulation may also be performedusing the results of the structural verification to determine themaximum number of time frames in which to verify the signals.

FIG. 11 illustrates an example of changed flip-flops after registerretiming according to an exemplary embodiment of the present disclosure.In this example, A and B are combinational logic elements. Registerretiming repositions flip-flops 1101 and 1102. The initial states offlip-flops 1101 and 1102 in the retimed circuit may be computed usingthe initial states of the flip-flops in the original circuit and theBoolean functionality of the combinational logic elements A and B.According to an embodiment of the present disclosure, verification maybe performed to confirm that the initial states of the changed(repositioned) flip-flops 1101 and 1102 are correct.

One important part of the verification procedure is identifyingappropriate signals to compare that should produce the same logic valuefor multiple time frames during simulation. For example, in FIG. 11,signals x and y should match for two time frames if the initial statesin the retimed circuit were computed correctly. The maximum value of theretiming labels computed for structural verification is used as thenumber of time frames required for simulation. Any incorrect initialstate computation will arise as compare point value mismatches whensimulating within these number of time frames, starting from the initialstates of the original and retimed circuits

FIG. 12 is a flow chart illustrating a method for verifying initialstate equivalence of changed flip-flops in a retimed circuit accordingto an embodiment of the present disclosure. The procedures illustratedin FIG. 12 may be used to implement procedure 450 (shown in FIG. 4).

At 1201, a maximum number of time frames which reflects a possiblevariation in functional behavior because of incorrect computation ofinitial states of changed flip-flops in a retimed circuit is identified.The number of time frames identified determines the upper bound of timeframes for which bounded sequential logic simulation should be performedin order to determine whether the initial states of changed flip-flopsin the retimed circuit were correctly computed. According to anembodiment of the present disclosure, the maximum absolute value of theretiming labels for the circuit (computed during structuralverification) is identified as the number of time frames during which avariation in functional behavior of the circuit caused by incorrectcomputation of initial states of the changed flip-flops may beexhibited. Any incorrect initial state computation may exhibit adifference in signal values of compare points, when simulating withinthese number of time frames.

At 1202, compare points are identified. The compare points representcorresponding points from the original and retimed circuit where signalvalues should be equivalent if the initial states of changed flip-flopswere correctly computed. According to an embodiment of the presentdisclosure, the retiming labels for the circuit (determined duringstructural verification) are used to identify the compare points.

For a retiming label, r, with a positive value, a compare point in theoriginal circuit is at an output of the node associated with theretiming label, and a corresponding compare point in the retimed circuitis at the output of the r^(th) flip-flop on the output of the node asindicated by the retiming label. For a retiming label, r, with anegative value, a compare point in the retimed circuit is at an outputof the node associated with the retiming label, and a correspondingcompare point in the original circuit is at the output of the r^(th)flip-flop at the output of the node indicated by the retiming label.

At 1203, sequential logic simulation is performed for a time frame.Given the known initial states of the flip-flops in the original andretimed circuits, sequential logic simulation is performed where signalvalues are identified at compare points of the circuits at a firstinitial time frame. This bounded sequential logic simulation uses3-valued simulation using {0, 1, X} values, where X represents anunknown value. It should be appreciated that procedure 1203 mayperformed for subsequent time frames.

At 1204, it is determined whether the values of the compare points inthe retimed circuit match the values of the corresponding compare pointsin the original circuit. According to an embodiment of the presentdisclosure, compare points in the retimed circuit match compare pointsat the original circuit if a signal value at the compare points of theretimed circuit match a signal value at corresponding compare points ofthe original circuit. If the compare points at the retimed circuits donot match the compare points at the original circuit, control proceedsto 1205. If the compare points at the retimed circuits match the comparepoints at the original circuit, control proceeds to 1206.

At 1205, an indication is generated that verification of initial statesof changed flip-flops was unsuccessful, and the mismatching comparepoints and the time-frame where the mismatches occurred is alsogenerated

At 1206, it is determined whether the compare points that werepreviously compared were at a last time frame for the bounded sequentiallogic simulation. According to an embodiment of the present disclosure,sequential logic simulation is bounded by the number of time framesidentified at 1201. If the compare points that were previously comparedwere not at the last time frame for the bounded sequential logicsimulation, control proceeds to 1203 where sequential simulation isperformed for a next time frame. This involves a clocking operation thattransfers the values from the D signal of every flip-flop to the Qsignal of the same flip-flop. If the compare points that were previouslycompared were at the last time frame for the bounded sequential logicsimulation, control proceeds to 1207.

At 1207, an indication is generated that verification of initial statesof changed flip-flops was successful.

It should be appreciated that the methodology for verifying initialstate equivalence of changed flip-flops in a retimed circuit as shown inFIG. 12 may be implemented using various techniques. For example, D andQ values for each flip-flop in the retimed circuit and the originalcircuit may be defined as random variables to be solved where signalvalues at identified compare points in the retimed circuit and originalcircuit are constrained to be equal. If a solution can be found for theD and Q values for each of the flip-flops in the retimed circuit and theoriginal circuit given the defined constraints, verification of initialstate equivalence of changed flip-flops in the retimed circuit issuccessful.

FIG. 13 is a flow chart illustrating a method for identifying comparepoints and performing bounded sequential logic simulation according toan embodiment of the present disclosure. The procedures illustrated inFIG. 13 may be used to implement procedures 1202-1207 (shown in FIG.12). It should be appreciated that prior to performing the procedures inFIG. 13, a first retiming graph is generated from an HDL description ofan original circuit and a second flow chart is generated from an HDLdescription of a retimed circuit. According to an embodiment of thepresent disclosure, the retiming graphs model combinational nodes asvertices with weights on edges representing a number of flip-flopsbetween corresponding combinational nodes represented by that edge. Thefirst and second retiming graphs may be traversed, and constraints maybe generated and solved in the manner described as follows. The initialstate values from the original and retimed circuits are also utilized.

At 1301, a first set of state variables is defined. The first set ofstate variables models initial states (initial values) of primary inputsand flip-flops in the original circuit and the retimed circuit.According to an embodiment of the present disclosure, flip-flops on anoutput edge of each combinational node that are common to all thefanouts of the combinational element output edge may be represented in astatic array. Flip-flops that are not common to all the fanouts of acombinational node output edge may be represented as a separate statevariables.

At 1302, a second set of state variables is defined. The second set ofstate variables models retiming labels. According to an embodiment ofthe present disclosure, an absolute value of the retiming labels mayalso be defined. The retiming labels defined at 1302 may be the retiminglabels computed at FIG. 5 during structural verification.

At 1303, a first set of random variables is defined. The first set ofrandom variables model D and Q values of the flip-flops in the originalcircuit and the retimed circuit. According to an embodiment of thepresent disclosure, the random variables may be represented as staticarrays or individual variables. A dummy array of size 1 may be createdfor edges in the circuits that do not have any flip-flops.

At 1304, a second set of random variables is defined. The second set ofrandom variables models values at an input and output of eachcombinational node in the original and retimed circuit.

At 1305, a third set of random variables is defined. According to anembodiment of the present disclosure, index variables are declared foreach combinational node that indexes into arrays representing values offlip-flops on an output of each of the combinational node.

At 1306, constraints are defined. According to an embodiment of thepresent disclosure, a first set of constraints is defined to enablelogic simulation. The first set of constraints includes constraints thattransfer values to the D and Q random variables from initial states,combinational elements, or primary inputs driving the flip-flops. Thefirst set of constraints also ensures that combinational node values arecomputed before the values are transferred. Constraints for thecombinational nodes also transfer values from a previous node in thecircuits and compute an output value for each combinational node usinginput values and Boolean functionality of the node. Ordering constraintsmay also be defined to ensure that computation proceeds in a topologicalordering where all inputs of a combinational element are computed beforethe output is computed. According to an embodiment of the presentdisclosure, a second set of constraints is defined to identify comparepoints corresponding to signals in the output of every combinationalnode in the original circuit and retimed circuit. Using the retiminglabel for each node, constraints appropriately compare combinationalnode output values to corresponding flip-flop values.

At 1307, solutions for the random variables are sought. According to anembodiment of the present disclosure, values for the random variablesare solved for given the state variables and constraints defined.Solutions for the random variables may be computed using an equationsolving routine or program.

At 1308, it is determined whether a solution for the random values isfound. If a solution for the random variables is found, control proceedsto 1309. This is the case when the constraints for all the comparepoints are satisfied for the current time frame. If a solution for therandom variables is not found, control proceeds to 1312.

At 1309, it is determined whether the solution for the random variablesfound were for a last time frame for bounded sequential logicsimulation. According to an embodiment of the present disclosure,sequential logic simulation is bounded by a number of time framesidentified by a maximum absolute value of retiming labels for the systemcomputed during structural verification. If the solution for the randomvariables found were not for the last time frame for the boundedsequential logic simulation, control proceeds to 1310. If the solutionfor the random variables found were for the last time frame for thebounded sequential logic simulation, control proceeds to 1311.

At 1310, a clock operation is performed to move data across time frames.The clocking operation transfers the D values on flip-flops of theoriginal circuit and the retimed circuit to the Q values which resultsin entering a new time frame. According to an embodiment of the presentdisclosure, three sets of values are used. In this embodiment, a Dvalues are moved to I values. I values are moved to Q values. Thistechnique allows for uniform treatment of initial state values andvalues of flip-flops between time frames. Essentially, a D value becomesthe initial state, I value, for the next time frame. The constraintsthemselves ensure that the Q value in a time frame is equal to theinitial state, I value, for that time frame.

At 1311, an indication is generated that verification of initial statesof changed flip-flops was successful.

At 1312, an indication is generated that verification of initial statesof changed flip-flops was unsuccessful.

The following example illustrates how the verification method describedwith reference to FIGS. 4, and 12-13 may be performed on the sequentialcircuit illustrated in FIG. 2A and the retiming graph illustrated inFIG. 3 according to an embodiment of the present disclosure.SystemVerilog is used as the programming language in this example. Itshould be appreciated, however, that other programming languages ortools may be used to implement the methodology described.

With reference to FIG. 2A, references by arrays and variables to aletter and number refer to an input from the referenced letter at gate(component) with the referenced number. For example, arrays andvariables with a1 in their names refer to the a input of gate G1. Theprimary output node O is modeled with array and variable names thatinclude out.

With reference to FIG. 13, at 1301, a first set of state variables isdefined to model initial states (initial values) of primary inputs andflip-flops in the original circuit and the retimed circuit. At 1302, asecond set of state variables is defined to models retiming labels andan absolute value of the retiming labels. These state variables may bedeclared as follows.

 // ORIGINAL CIRCUIT INITIAL STATE VARIABLES bit[1:0] o_in1, o_in2; //FF initial states bit[1:0] o_in1_ff_i[1];  bit[1:0] o_in2_ff_i[1]; bit[1:0] o_z6_ff_i[1];  // FF 4 is not common to all fanouts of gate 6,so have a separate variable bit[1:0] o_i3;  // RETIMED CIRCUIT INITIALSTATE VARIABLES // Input/outputs bit[1:0] r_in1, r_in2;  // FF initialstates bit[1:0] r_z3_ff_i[1];  bit[1:0] r_z4_ff_i[1];  bit[1:0]r_z5_ff_i[1];  // FF 4 is not common to all fanouts of gate 6, so have aseparate variable bit[1:0] r_i3;  integer r1, r2, r3, r4, r5, r6; integer a_r1, a_r2, a_r3, a_r4, a_r5, a_r6;

The state variables may be initialized as follows.

task init_state_variables( );   // Original circuit   o_in1 = 2;   o_in2= 2;   o_in1_ff_i[0] = 1;   o_in2_ff_i[0] = 1;   o_z6_ff_i[0] = 0;  o_i3 = 0;   // Retimed circuit   r_in1 = 2;   r_in2 = 2;  r_z3_ff_i[0] = 1;   r_z4_ff_i[0] = 1;   r_z5_ff_i[0] = 0;   r_i3 = 0;  r1 = 0;   r2 = 0;   r3 = 0;   r4 = 0;   r5 = 0;   r6 = 0;   a_r1 = 0;  a_r2 = 0;   a_r3 = 0;   a_r4 = 0;   a_r5 = 0;   a_r6 = 0;  endtasktask init_rs(foo bar);   r1 = bar.r1;   r2 = bar.r2;   r3 = bar.r3;   r4= bar.r4;   r5 = bar.r5;   r6 = bar.r6;  //  Also  store  absolute  values,  for  easy  indexing later   a_r1 =(r1 >= 0)?r1:−r1;   a_r2 = (r2 >= 0)?r2:−r2;   a_r3 = (r3 >= 0)?r3:−r3;  a_r4 = (r4 >= 0)?r4:−r4;   a_r5 = (r5 >= 0)?r5:−r5;   a_r6 = (r6 >=0)?r6:−r6;  endtask

At 1303, a first set of random variables is defined to model D and Qvalues of the flip-flops in the original circuit and the retimedcircuit. At 1304, a second set of random variables is defined to modelvalues at inputs and output of each combinational node in the originaland retimed circuit. At 1305, a third set of random variables is definedthat model index variables for each combinational node that indexes intoarrays representing values of flip-flops on an output of each of thecombinational node. These state variables may be declared as follows.

 // ORIGINAL CIRCUIT RANDOM VARIABLES  // FF input/outputs  // FFs thatare common to all fanouts of a gate are modeled as array variables;  //all other FFs that are specific to each fanout need to be modeledseparately  // as independent random variables.  rand bit[1:0]o_in1_ff_d[1];  rand bit[1:0] o_in1_ff_q[1];  rand bit[1:0]o_in2_ff_d[1];  rand bit[1:0] o_in2_ff_q[1];  rand bit[1:0]o_z6_ff_d[1];  rand bit[1:0] o_z6_ff_q[1];  rand bit[1:0] o_d3, o_q3; // Dummy arrays for nodes that don't have flip-flops  rand bit[1:0]o_z1_ff_q[1];  rand bit[1:0] o_z2_ff_q[1];  rand bit[1:0] o_z3_ff_q[1]; rand bit[1:0] o_z4_ff_q[1];  rand bit[1:0] o_z5_ff_q[1];  //Combinational logic  rand bit[1:0] o_a1, o_b1, o_z1, o_a2, o_b2, o_z2,o_a3, o_b3, o_z3, o_a4, o_b4, o_z4, o_a5, o_b5, o_z5, o_a6, o_b6, o_z6; // RETIMED CIRCUIT RANDOM VARIABLES  // FF input/outputs  // FFs thatare common to all fanouts of a gate are modeled as array variables;  //all other FFs that are specific to each fanout need to be modeledseparately  // as independent random variables.  rand bit[1:0]r_z3_ff_d[1];  rand bit[1:0] r_z3_ff_q[1];  rand bit[1:0] r_z4_ff_d[1]; rand bit[1:0] r_z4_ff_q[1];  rand bit[1:0] r_z5_ff_d[1];  rand bit[1:0]r_z5_ff_q[1];  rand bit[1:0] r_d3, r_q3  // Dummy arrays for nodes thatdon't have flip-flops  rand bit[1:0] r_z1_ff_q[1];  rand bit[1:0]r_z2_ff_q[1];  rand bit[1:0] r_z6_ff_q[1];  // Combinational logic  randbit[1:0] r_a1, r_b1, r_z1, r_a2, r_b2, r_z2, r_a3, r_b3, r_z3, r_a4,r_b4, r_z4, r_a5, r_b5, r_z5, r_a6, r_b6, r_z6;  // INDEX RANDOMVARIABLES  rand integer r1_index, r2_index, r3_index, r4_index,r5_index, r6_index;

At 1306, constraints are defined. According to an embodiment of thepresent disclosure, a first set of constraints is defined to enablelogic simulation, and a second set of constraints is defined to identifycompare points corresponding to signals in the output of everycombinational node in the original circuit and retimed circuit. Theconstraints may be defined as follows.

The exemplary constraint blocks illustrated above perform the followingoperations. For each combinational gate (node), a determination is madeas to whether a structural verifier required any moves of flip-flopsacross the node. A non-zero retiming label on a node reflects that someflip-flop movement across the node was required to retime the retimedcircuit back to the original circuit. If the retiming label has apositive value, the retimer moved that number of flip-flops from theinputs of the node to the output of the node. In such a case, if thoseflip-flops are still in the output of the node in the retimed circuit(because they did not get further forward retimed), a compare point maybe derived that corresponds to the rth flip-flop on the output of thenode in the retimed circuit and the output node in the original circuit.If the retiming label has a negative value, the retimer moved thatnumber of flip-flops from the output of the node in the original circuitto its inputs. In such a case, if the original circuit had r flip-flopson the output of the node, a compare point may be derived thatcorresponds to the output of the node in the retimed circuit and thesignal corresponding to the rth flip-flop on the output of the originalcircuit.

At 1307, solutions for the random variables are sought. According to anembodiment of the present disclosure, values for the random variablesare solved for given the state variables and constraints defined.Solutions for the random variables may be computed using an equationsolving routine or program.

At 1308, if a solution for the random variables is found controlproceeds to 1309. This represents the case when all compare pointconstraints are satisfied for the current time frame. This means thevalues of compare points are consistent in the current time frame. If asolution for the random variables is not found, control proceeds to1312.

At 1309, it is determined whether the solution for the random variablesfound were for a last time frame for bounded sequential logicsimulation. According to an embodiment of the present disclosure,sequential logic simulation is bounded by a number of time framesidentified by a maximum absolute value of retiming labels for thesystem. If the solution for the random variables found were not for thelast time frame for the bounded sequential logic simulation, controlproceeds to 1310. If the solution for the random variables found werefor the last time frame for the bounded sequential logic simulation,control proceeds to 1311.

At 1310, a clock operation is performed to move data across time frames.The clocking operation transfers the D values on flip-flops of theoriginal circuit and the retimed circuit to the Q values which resultsin entering a new time frame. To enable uniform treatment of initialstates, the clocking operation transfers the D value to an initialstate, I value. The constraints themselves ensure that the initial stateI values are transferred to the Q variables. The clock operation may beimplemented with the following.

task clock_it( );  // Original circuit  foreach(o_in1_ff_i[i]) begin  o_in1_ff_i[i] = o_in1_ff_d[i];  end  foreach(o_in2_ff_i[i]) begin  o_in2_ff_i[i] = o_in2_ff_d[i];  end  foreach(o_z6_ff_i[i]) begin  o_z6_ff_i[i] = o_z6_ff_d[i];  end  o_i3 = o_d3;  // Retimed circuit foreach(r_z3_ff_i[i]) begin    r_z3_ff_i[i] = r_z3_ff_d[i];  end foreach(r_z4_ff_i[i]) begin    r_z4_ff_i[i] = r_z4_ff_d[i];  end foreach(r_z5_ff_i[i]) begin    r_z5_ff_i[i] = r_z5_ff_d[i];  end  r_i3= r_d3; endtask

At 1311, an indication is generated that verification of initial statesof changed flip-flops was successful.

At 1312, an indication is generated that verification of initial statesof changed flip-flops was unsuccessful.

It should be appreciated that the compare points described withreference to the present disclosure may also be used to perform boundedsequential logic simulation in a different manner. For example, thecompare points may be used in a simulator such as VCS, ModelSim, orother simulator without using constraint language. The compare pointsmay then be modeled as assertions during the simulation.

FIGS. 1, 4-5, 8-10, and 12-13 are flow charts that illustrateembodiments of the present disclosure. The procedures described in thesefigures may be performed by an EDA tool implemented by a computersystem. Some of the techniques illustrated may be performedsequentially, in parallel or in an order other than that which isdescribed and that the procedures described may be repeated. It isappreciated that not all of the techniques described are required to beperformed, that additional techniques may be added, and that some of theillustrated techniques may be substituted with other techniques.

FIG. 14 is a block diagram of an exemplary computer system 1400 in whichan example embodiment of the present disclosure resides. The computersystem 1400 includes a processor 1410 that process data signals. Theprocessor 1410 is coupled to a bus 1401 or other switch fabric thattransmits data signals between processor 1410 and other components inthe computer system 1400. The computer system 1400 includes a memory1420. The memory 1420 may store instructions and code represented bydata signals that may be executed by the processor 1410. A data storagedevice 1430 is also coupled to the bus 1401.

A network controller 1440 is coupled to the bus 1401. The networkcontroller 1440 may link the computer system 1400 to a network ofcomputers (not shown) and supports communication among the machines. Adisplay device controller 1450 is coupled to the bus 1401. The displaydevice controller 1450 allows coupling of a display device (not shown)to the computer system 1400 and acts as an interface between the displaydevice and the computer system 1400. An input interface 1460 is coupledto the bus 1401. The input interface 1460 allows coupling of an inputdevice (not shown) to the computer system 1400 and transmits datasignals from the input device to the computer system 1400.

A system designer 1421 may reside in the memory 1420 and be executed bythe processor 1410. The system designer 1421 may operate to design asystem by performing synthesis, placement, and routing on the system.The system designer 1421 may also perform register retiming andverification of the retimed system. According to an embodiment of thepresent disclosure, verification may include verifying structuralcorrectness of the retimed circuit, verifying initial states equivalenceof unchanged flip-flops in the retimed circuit, and verifying initialstates equivalence of changed flip-flops in the retimed circuit.

FIG. 15 illustrates a system designer 1500 according to an embodiment ofthe present disclosure. The system designer 1500 may be an EDA tool fordesigning a system on a target device such as an FPGA, structuredapplication-specific integrated circuit (ASIC), or other circuitry. FIG.15 illustrates modules implementing an embodiment of the system designer1500. According to one embodiment, the modules represent softwaremodules and system design may be performed by a computer system such asthe one illustrated in FIG. 14 executing sequences of instructionsrepresented by the modules shown in FIG. 15. Execution of the sequencesof instructions causes the computer system to support system design aswill be described hereafter. In alternate embodiments, hard-wirecircuitry may be used in place of or in combination with softwareinstructions to implement embodiments of present disclosure. Thus,embodiments of present disclosure are not limited to any specificcombination of hardware circuitry and software.

The system designer 1500 includes a designer manager 1510. The designermanager 1510 is connected to and transmits data between the componentsof the system designer 1500.

The system designer 1500 includes a synthesis unit 1520 that generates alogic design of a system to be implemented on the target device.According to an embodiment of the system designer 1500, the synthesisunit 1520 takes a conceptual HDL design definition and generates anoptimized logical representation of the system. The optimized logicalrepresentation of the system generated by the synthesis unit 1520 mayinclude a representation that has a reduced number of functional blocksand registers, such as logic gates and logic elements, required for thesystem. Alternatively, the optimized logical representation of thesystem generated by the synthesis unit 1520 may include a representationthat has a reduced depth of logic and that generates a lower signalpropagation delay.

The synthesis unit 1520 also performs technology mapping. Technologymapping involves determining how to implement the functional blocks andregisters in the optimized logic representation utilizing specificresources such as cells on a target device thus creating an optimized“technology-mapped” netlist. The technology-mapped netlist illustrateshow the resources (cells) on the target device are utilized to implementthe system. In an embodiment where the target device is an FPGA, thetechnology-mapped netlist may include cells such as logic array blocks(LABs), registers, memory blocks, digital signal processing (DSP)blocks, input output (IO) elements or other components.

The system designer 1500 includes a placement unit 1530 that processesthe optimized technology-mapped netlist to produce a placement for eachof the functional blocks. The placement identifies which components orareas on the target device are to be used for specific functional blocksand registers.

The system designer 1500 includes a routing unit 1540 that determinesthe routing resources on the target device to use to provideinterconnection between the components implementing functional blocksand registers of the logic design.

The system designer 1500 includes a retiming unit 1550 that improves theperformance of sequential circuits in the system by repositioningflip-flops (registers) without changing the combinational path(s)between flip-flops and/or input outputs (IOs) that have the worst delay.The retiming unit 1550 may perform the optimizations described withreference to FIGS. 2A-2C.

The system designer 1500 includes a verification unit 1560 that confirmswhether a retimed design for the system is equivalent to the originaldesign. According to an embodiment of the present disclosure, theverification unit 1560 verifies that a circuit before retiming isfunctionally equivalent to the circuit after retiming. As such, theverification unit 1560 uses the netlist before and after retiming andthe initial states of all flip-flops in the original and retimedcircuits. The verification unit 1560 includes a structural correctnessverifier unit 1561 that determines whether a retimed circuit isstructurally correct. The structural correctness verifier unit 1561 mayinclude components or modules that implement procedures disclosed withreference to FIGS. 1, 4, 5, 17, 18, 21, and 22. The verification unit1560 includes an unchanged flip-flop initial state equivalence unit 1562that determines whether initial states of unchanged flip-flops inretimed circuits are equivalent. The unchanged flip-flop initial stateequivalence unit 1562 may include components or modules that implementprocedures disclosed with reference to FIGS. 1, 4, and 8-10. Theverification unit 1560 includes a changed flip-flop initial stateequivalence unit 1563 that determines whether initial states of changedflip-flops in retimed circuits are equivalent. The changed flip-flopinitial state equivalence unit 1563 may include components or modulesthat implement procedures disclosed with reference to FIGS. 1, 4, and12-13. According to an embodiment of the present disclosure, theconstraints minimization procedure described may also be combined withprocedures 1562 and 1563. Similarly, the handling of multiple clocks mayalso be combined with procedures 1562 and 1563, where for each clock,all procedures of rewind verification (structural verification,verification of unchanged initial states, verification of changedinitial states) are performed.

It should be appreciated that the register retiming unit 1550 mayperform register retiming and the verification unit 1560 may performverification during and/or after synthesis, placement, and/or routing.

It should be appreciated that embodiments of the present disclosure maybe provided as a computer program product, or software, that may includea computer-readable or machine-readable medium having instructions. Theinstructions on the computer-readable or machine-readable medium may beused to program a computer system or other electronic device. Themachine-readable medium may include, but is not limited to, floppydiskettes, optical disks, CD-ROMs, and magneto-optical disks or othertype of media/machine-readable medium suitable for storing electronicinstructions. The techniques described herein are not limited to anyparticular software configuration. They may find applicability in anycomputing or processing environment. The terms “computer-readablemedium” or “machine-readable medium” used herein shall include anymedium that is capable of storing or encoding a sequence of instructionsfor execution by the computer and that cause the computer to perform anyone of the methods described herein. Furthermore, it is common in theart to speak of software, in one form or another (e.g., program,procedure, process, application, module, unit, logic, and so on) astaking an action or causing a result. Such expressions are merely ashorthand way of stating that the execution of the software by aprocessing system causes the processor to perform an action to produce aresult.

FIG. 16 illustrates a device 1600 that may be used to implement a targetdevice according to an embodiment of the present disclosure. The device1600 is a field programmable gate array (FPGA) that includes a pluralityof logic-array blocks (LABs). According to an embodiment of the presentdisclosure, the device 1600 may be implemented on a single integratedcircuit. Each LAB may be formed from a plurality of logic blocks, carrychains, LAB control signals, look up table (LUT) chain, and registerchain connection lines. A logic block is a small unit of logic providingefficient implementation of user logic functions. A logic block includesone or more combinational cells, where each combinational cell has asingle output, and registers. According to one embodiment of the presentdisclosure, the logic block may operate similarly to a logic element(LE), such as those found in the Stratix or Cyclone devices manufacturedby Altera Corporation, or a combinational logic block (CLB) such asthose found in Virtex devices manufactured by Xilinx Inc. In thisembodiment, the logic block may include a four input LUT with aconfigurable register. According to an alternate embodiment of thepresent disclosure, the logic block may operate similarly to an adaptivelogic module (ALM), such as those found in Stratix devices manufacturedby Altera Corporation. LABs are grouped into rows and columns across thedevice 1600. Columns of LABs are shown as 1611-1616. It should beappreciated that the logic block may include additional or alternatecomponents.

The device 1600 includes memory blocks. The memory blocks may be, forexample, dual port random access memory (RAM) blocks that providededicated true dual-port, simple dual-port, or single port memory up tovarious bits wide at up to various frequencies. The memory blocks may begrouped into columns across the device in between selected LABs orlocated individually or in pairs within the device 1600. Columns ofmemory blocks are shown as 1621-1624.

The device 1600 includes digital signal processing (DSP) blocks. The DSPblocks may be used to implement multipliers of various configurationswith add or subtract features. The DSP blocks include shift registers,multipliers, adders, and accumulators. The DSP blocks may be groupedinto columns across the device 1600 and are shown as 1631.

The device 1600 includes a plurality of input/output elements (IOEs)1640. Each IOE feeds an IO pin (not shown) on the device 1600. The IOEs1640 are located at the end of LAB rows and columns around the peripheryof the device 1600. Each IOE may include a bidirectional IO buffer and aplurality of registers for registering input, output, and output-enablesignals.

The device 1600 may include routing resources such as LAB localinterconnect lines, row interconnect lines (“H-type wires”), and columninterconnect lines (“V-type wires”) (not shown) to route signals betweencomponents on the target device. Although the exemplary device 1600illustrated in FIG. 16 is a FPGA, the present disclosure may be appliedto ASICs and to any general digital circuit implementation.

The following examples pertain to further embodiments. In oneembodiment, a method for performing rewind functional verificationincludes identifying state variables that model a number of registers oneach edge of a retiming graph for an original design and a retimeddesign. Random variables are identified that model retiming labelsrepresenting a number and direction of register movement relative to anode on a retiming graph for the retimed design. A retiming constraintis identified for each edge on the retiming graph for the design,wherein the retiming constraint reflects a relationship between thestate variables and the random variables. A random variable that modelsa retiming label at a source of an edge is substituted for a randomvariable that models a retiming label at a sink of the edge when anumber of registers on the edge is unchanged after register retiming.

In a further embodiment, the method further includes substituting therandom variable that models the retiming label at the source of the edgefor another random variable that models a retiming label at a nodeconnected to the sink of the edge when a number of registers on an edgeconnecting the node to the sink is unchanged after the registerretiming.

In a further embodiment, the method further includes substituting therandom variable that models the retiming label at the source of the edgefor a further random variable that models a retiming label at a furthernode connected to the node when a number of registers on an edgeconnecting the further node to the node is unchanged after the registerretiming.

In a further embodiment, the method wherein the substituting of therandom variable that models the retiming label at the source isperformed recursively for other connected nodes.

In a further embodiment, the method further includes removing aredundant constraint in response to the substituting.

In a further embodiment, the method further includes determining whetherthe retimed design is structurally correct in response to identifyingsolutions for remaining random variables.

In a further embodiment, the method further includes identifyingsolutions for the random variables without the substituting and theremoving in response to determining the retimed design is structurallyincorrect.

In a further embodiment, the method further includes identifying an edgeassociated with the retimed design being structurally incorrect inresponse to identifying solutions for the random variables.

In a further embodiment, a non-transitory computer-readable mediumhaving sequences of instructions, the sequences of instructionsincluding instructions which, when executed, causes a processor toperform the method of any one of the previously described embodiments.

In a further embodiment, an apparatus comprising means to perform amethod as claimed in any one of the previously described embodiments.

In another embodiment, a method for performing rewind functionalverification includes identifying random variables that model retiminglabels representing a number and direction of register movement relativeto a node on a retiming graph for a retimed design. An edge on aretiming graph is identified for an original design and a retimeddesign. Random variables for nodes associated with the edge aredesignated to be in an equivalence class in response to determining thata number of registers on the edge is unchanged after register retiming.A next edge on the retiming graph for the original design and theretimed design that is connected to a node having a random variable inthe equivalence class is identified. Random variables for nodesassociated with the next edge are designated to be in the equivalenceclass in response to determining that a number of registers on the nextedge is unchanged after the register retiming. The random variables inthe equivalence class are substituted with a new single variable. It isdetermined whether the retimed design is structurally correct inresponse to identifying solutions for remaining random variables afterthe substituting.

In a further embodiment, the method further includes identifying statevariables that model a number of registers on each edge of the retiminggraph for the original design and a retimed design, identifying aretiming constraint for each edge on the retiming graph for the design,wherein the retiming constraint reflects a relationship between thestate variables and the random variables.

In a further embodiment, the method further includes removing aredundant constraint in response to the substituting.

In a further embodiment, the method further includes identifyingsolutions for the random variables without the substituting and theremoving in response to determining the retimed design is structurallyincorrect.

In a further embodiment, the method further includes identifying an edgeassociated with the retimed design being structurally incorrect inresponse to identifying solutions for the random variables.

In a further embodiment, a non-transitory computer-readable mediumhaving sequences of instructions, the sequences of instructionsincluding instructions which, when executed, causes a processor toperform the method of any one of the previously described embodiments.

In a further embodiment, an apparatus comprising means to perform amethod as claimed in any one of the previously described embodiments.

In another embodiment, a system designer for performing rewindfunctional verification, includes means for identifying random variablesthat model retiming labels representing a number and direction ofregister movement relative to a node on a retiming graph for a retimeddesign. The system designer includes means for identifying an edge on aretiming graph for an original design and a retimed design. The systemdesigner includes means for designating random variables for nodesassociated with the edge to be in an equivalence class in response todetermining that a number of registers on the edge is unchanged afterregister retiming. The system designer includes means for identifying anext edge on the retiming graph for the original design and the retimeddesign that is connected to a node having a random variable in theequivalence class. The system designer includes means for designatingrandom variables for nodes associated with the next edge to be in theequivalence class in response to determining that a number of registerson the next edge is unchanged after the register retiming. The systemdesigner includes means for substituting the random variables in theequivalence class with a new single variable. The system designerincludes means for determining whether the retimed design isstructurally correct in response to identifying solutions for remainingrandom variables after the substituting.

In another embodiment, a method for designing a system on a targetdevice includes performing register retiming on an original design forthe system to generate a retimed design, and verifying whether theretimed design is structurally correct by performing a plurality ofiterations of register retiming on the retimed design, wherein eachiteration accounts for the retiming of registers in the system driven bya different clock.

In a further embodiment, performing register retiming on the retimeddesign comprises for each of the plurality of iterations, defining aweight for each edge on a retiming graph for the original design and aretiming graph for the retimed design, wherein the weight represents anumber of registers on the edge driven by a clock associated with aniteration.

In a further embodiment, the method further includes for each of theplurality of iterations, identifying state variables that model thenumber of registers on the edge of the retiming graph for the originaldesign and for the retimed design corresponding to registers driven by acurrent clock associated with a current iteration.

In a further embodiment, the method further includes for each of theplurality of iterations, identifying random variables that modelretiming labels representing a number and direction of register movementrelative to a node on a retiming graph for the retimed design.

In a further embodiment, the method further includes for each of theplurality of iterations, identifying a retiming constraint for each edgeon the retiming graph for the design, wherein the retiming constraintreflects a relationship between the state variables and the randomvariables.

In a further embodiment, the method further includes for each of theplurality of iterations, identifying a multi-clock constraint for eachedge on the retiming graph for the design, wherein the multi-clockconstraint prevents a register driven by a first clock to be moved pasta register driven by any other clock.

In a further embodiment, the method further includes determining thatthe retimed design is structurally correct in response to findingsolutions for the random variables.

In a further embodiment, the method further includes determining thatthe retimed design is structurally correct in response to determiningthat performing the plurality of iterations of register retiming on theretimed design results in the original design.

In a further embodiment, the method further includes identifying alargest absolute value for random variables representing the retiminglabels.

In a further embodiment, a non-transitory computer-readable mediumhaving sequences of instructions, the sequences of instructionsincluding instructions which, when executed, causes a processor toperform the method of any one of the previously described embodiments.

In a further embodiment, an apparatus comprising means to perform amethod as claimed in any one of the previously described embodiments.

In another embodiment, a method for performing rewind functionalverification includes for each clock driving an original design and aretimed design, generating a retiming graph for the original design anda retiming graph for the retimed design where a weight is identified foreach edge that represents a number of registers driven on the edge bythe clock. State variables are identified that model a number ofregisters on each edge of the retiming graphs. Random variables areidentified that model retiming labels representing a number anddirection of register movement relative to a node on the retiminggraphs. A retiming constraint is identified for each edge on theretiming graphs, wherein the retiming constraint reflects a relationshipbetween the state variables and the random variables. A multi-clockconstraint is identified for each edge on the retiming graphs, whereinthe multi-clock constraint prevents a register driven by a first clockto be moved past a register driven by any other clock. It is determinedwhether the retimed design is structurally correct in response toidentifying solutions for the random variables.

In a further embodiment, the method includes generating an indication ifthe retimed design is structurally incorrect.

In a further embodiment, the method further includes substituting arandom variable that models a retiming label at a source of an edge fora random variable that models a retiming label at a sink of the edgewhen a number of registers on the edge is unchanged after registerretiming.

In a further embodiment, the method further includes substituting therandom variable that models the retiming label at the source of the edgefor another random variable that models a retiming label at a nodeconnected to the sink of the edge when a number of registers on an edgeconnecting the node to the sink is unchanged after the registerretiming.

In a further embodiment, the method further includes removing aredundant constraint in response to the substituting.

In a further embodiment, a non-transitory computer-readable mediumhaving sequences of instructions, the sequences of instructionsincluding instructions which, when executed, causes a processor toperform the method of any one of the previously described embodiments.

In a further embodiment, an apparatus comprising means to perform amethod as claimed in any one of the previously described embodiments.

In another embodiment, a system designer for performing rewindfunctional verification includes for each clock driving an originaldesign and a retimed design, means for generating a retiming graph forthe original design and a retiming graph for the retimed design where aweight is identified for each edge that represents a number of registersdriven on the edge by the clock. The system designer includes means foridentifying state variables that model a number of registers on eachedge of the retiming graphs. The system designer includes means foridentifying random variables that model retiming labels representing anumber and direction of register movement relative to a node on theretiming graphs. The system designer includes means for identifying aretiming constraint for each edge on the retiming graphs, wherein theretiming constraint reflects a relationship between the state variablesand the random variables. The system designer includes means foridentifying a multi-clock constraint for each edge on the retiminggraphs, wherein the multi-clock constraint prevents a register driven bya first clock to be moved past a register driven by any other clock. Thesystem designer includes means for determining whether the retimeddesign is structurally correct in response to identifying solutions forthe random variables.

In the foregoing specification, embodiments of the disclosure have beendescribed with reference to specific exemplary embodiments thereof. Itwill, however, be evident that various modifications and changes may bemade thereto without departing from the broader spirit and scope of theembodiments of the disclosure. The specification and drawings are,accordingly, to be regarded in an illustrative rather than restrictivesense.

What is claimed is:
 1. A method for designing a system on a targetdevice, the method comprising: performing register retiming on anoriginal design to generate a retimed design of the system; identifyinga maximum number of time frames that reflects a variation in functionalbehavior because of incorrect computation of initial states of changedflip-flops in the retimed design; identifying compare points in theoriginal design and the retimed design where signal values reflectinitial states of one or more flip-flops; performing a boundedsequential logic simulation within a time frame, wherein the maximumnumber of time frames determines an upper bound of the time frame; anddetermining whether the changed flip-flops in the retimed design haveinitial states that are correct by comparing signal values at thecompare points from the bounded sequential logic simulation.
 2. Themethod of claim 1 further comprising identifying the compare points fromretiming labels that reflect a number and a direction of registermovement relative to a node in the retimed design.
 3. The method ofclaim 1, wherein the maximum number of time frames for performing thebounded sequential logic simulation is determined from a maximumabsolute value of retiming labels that reflect a number and a directionof register movement relative to a node in the retimed design.
 4. Themethod of claim 1, wherein a changed flip-flop is a flip-flop that hasbeen re-positioned from the original design.
 5. The method of claim 1,wherein an initial state is a state of a register at power-up.
 6. Themethod of claim 1, wherein the determining whether the changedflip-flops in the retimed design have initial states that are correct isperformed in response to determining that unchanged flip-flops in theretimed design have initial states that are correct.
 7. The method ofclaim 1 further comprising generating an indication of the correctnessof the initial states of the changed flip-flops.
 8. The method of claim1 further comprising: generating a data file that describes the retimeddesign in response to determining that the initial states are correct;and programming the target device with the data file to physicallytransform components on the target device to implement the system.
 9. Amethod for designing a system on a target device, the method comprising:performing register retiming on an original design to generate a retimeddesign; determining whether the retimed design is structurally correctby generating a retiming graph from a description of the original designthat models combinational nodes as vertices with weights on edgesrepresenting a number of registers between the combinational nodes;determining whether unchanged registers in the retimed design haveinitial states that are correct in response to determining whether theretimed design is structurally correct; and determining whether changedregisters in the retimed design have initial states that are correct inresponse to determining that the unchanged registers in the retimeddesign have initial states that are correct.
 10. The method of claim 9,wherein the determining whether the retimed design is structurallycorrect comprises: performing retiming on the retimed design; anddetermining whether performing register retiming on the retimed designresults in the original design.
 11. The method of claim 10, wherein theperforming retiming on the retimed design comprises identifying retiminglabels that represent a number and a direction of a register movementrelative to a node in the retimed design.
 12. The method of claim 9,wherein the determining whether unchanged registers in the retimeddesign have initial states that are correct further comprisesidentifying one or more unchanged registers of the unchanged registersin the retimed design by: identifying indices for each edge in theoriginal design and in the retimed design; and utilizing the indices todetermine whether an unchanged register resides on an edge.
 13. Themethod of claim 12, wherein the identifying indices for each edgecomprises: identifying a left index for each edge on the original designand the retimed design; and identifying a right index for each edge onthe original design and the retimed design, wherein values for the leftindex and the right index for each edge reflect whether one or moreunchanged registers resides on each edge.
 14. The method of claim 9,wherein the determining whether changed registers in the retimed designhave initial states that are correct comprises: identifying comparepoints in the original design and the retimed design that reflectbehavior of one or more changed registers of the changed registers;performing a bounded sequential logic simulation with a time framedetermined from a maximum absolute value of a retiming label for thesystem; and comparing signal values at the compare points from thebounded sequential logic simulation.
 15. The method of claim 14, whereinthe identifying compare points comprises using retiming labels thatreflect a number and a direction of a register movement relative to anode in the retimed design.
 16. The method of claim 14, wherein thecomparing signal values at the compare points comprises: modeling thecompare points as constraints; solving for the constraints at the timeframe determined; and determining that the changed registers in theretimed design have initial states that are correct if the constraintsare solvable at the time frame determined.
 17. The method of claim 14,wherein the bounded sequential logic simulation is performed using aconstraint solver.
 18. The method of claim 9 further comprising:generating a data file that describes the retimed design in response todetermining that the initial states are correct; and programming thetarget device with the data file to physically transform components onthe target device to implement the system.
 19. A system designer,comprising: a register retiming unit that performs register retiming onan original design for the system to generate a retimed design; and averification unit that identifies unchanged flip-flops and verifiesinitial state equivalence of the unchanged flip-flops between theoriginal design and the retimed design using state variables that modelretiming labels, wherein each of the retiming labels identifies a numberof flip-flops that move across a vertex, and wherein at least one of theregister retiming unit and the verification unit is implemented by aprocessor.
 20. The system designer of claim 19, wherein the verificationunit verifies whether the retimed design is structurally correct byperforming register retiming on the retimed design.
 21. The systemdesigner of claim 19, wherein the verification unit identifies changedflip-flops and verifies initial state equivalence of changed flip-flopsbetween the original design and the retimed design.